Longspined porcupinefish (Diodon holocanthus)

Understand this large marine fish before choosing it

The Longspined porcupinefish (Diodon holocanthus) is a marine porcupinefish in the family Diodontidae. It has a circumtropical distribution across parts of the Atlantic, Indian and Pacific oceans, and FishBase records a maximum length of 50 cm total length for male or unsexed fish. It suits a keeper prepared to plan a mature saltwater aquarium around that full adult figure, maintain marine water consistently and choose companions around its natural feeding behaviour.

In nature, this benthopelagic fish occurs from shallow reefs and rocky ground to open muddy or soft-bottom habitats. FishBase usually records it at depths of 2–35 m, within a wider recorded depth range of 2–200 m. The species is described as solitary and a relatively poor swimmer, although it sometimes forms groups and young or sub-adult fish may form small groups. Juveniles are pelagic until roughly 6–9 cm and are often associated with floating Sargassum rafts.

The Long-spine porcupinefish summary also records the common names balloonfish, freckled porcupinefish, porcupine puffer and porcupine pufferfish. These names all refer here to the same species in Diodontidae. It is assessed as Least Concern on the IUCN Red List.

Know what one purchase means

One purchase supplies one live Longspined porcupinefish. The 50 cm figure is the maximum recorded adult length; plan the permanent aquarium around it whatever size the fish is on arrival. Build the long-term plan before purchase rather than treating the first aquarium as temporary accommodation without a defined route to an adult-sized system.

Sex is guaranteed only when the selected option explicitly states male or female. This Longspined porcupinefish listing has no sex-guaranteed option.

The gallery images are representative of this type of animal rather than photographs of the exact animal supplied, so colours can look slightly different from the live animal.

Use the identity, adult size and care requirements as the selection criteria rather than relying on apparent colour, sex or life stage in an image. If you are comparing different focal animals for an established aquarium, browse the centrepiece fish collection and assess each candidate against the aquarium’s dimensions, chemistry and inhabitants.

Use the decisive planning values

Plan for a fish that can reach 50 cm total length and maintain a fish-only marine aquarium at 24–27°C, pH 8.0–8.2 and salinity of 1.017–1.023 SG, equivalent to 23–30 ppt. The OATA marine aquarium sheet gives these as broad group guidance for fish-only systems. Keep ammonia and nitrite at zero mg per litre and nitrate at no more than 20 mg per litre.

Judge an aquarium by its usable interior space for a 50 cm fish rather than by its litre capacity alone. Start with the recorded adult length, then consider the usable interior space after rockwork, the demands placed on filtration and the adult requirements of every other inhabitant. OATA advises choosing as large an aquarium as reasonably possible because a larger body of water is more stable and less susceptible to changes in temperature and pH.

Plan the aquarium around the adult size

Use 50 cm total length as the decisive space figure from the beginning. OATA advises buying, within reason, as large an aquarium as possible, ideally one that can accommodate the chosen fish once fully grown. Larger systems contain more water and are consequently more stable against variations in temperature and pH than small aquaria.

Assess the complete system

• Start with the adult fish: compare the 50 cm record with the aquarium’s usable interior space, not merely its stated volume.
• Allow for the aquascape: rockwork and substrate occupy part of the interior, so assess the space remaining once the intended layout is in place.
• Include every adult inhabitant: stocking capacity depends on body size, species requirements, water parameters and compatibility, rather than a simple count of fish.
• Plan filtration at the same time: the aquarium must have a mature biological filter capable of maintaining zero ammonia and zero nitrite.

Place the aquarium on a flat, level surface or stand capable of taking the weight of the filled system indefinitely. Keep it out of direct sunlight and away from heat sources, draughts, loud noises, vibration and sudden movement. These choices reduce avoidable environmental variation and disturbance.

If the fish starts in a smaller aquarium, you must be prepared to move it to a bigger set-up as it grows. A sound plan identifies the adult accommodation before the animal is introduced. If a future larger system forms part of the plan, define its dimensions, support, equipment and maturation route in advance rather than relying on growth to set the deadline.

A useful final check is simple: the proposed aquarium should still make sense when the animal is considered as a 50 cm marine fish, the rockwork has been installed and all intended inhabitants are assessed at their adult requirements. If the plan starts with a smaller aquarium, decide the bigger set-up before purchase.

Shape a practical marine habitat

Build a marine environment with refuge, stable equipment and usable open space. FishBase records this species across shallow reefs, rocky substrata, open muddy areas and rich soft-bottom and coral-reef habitats. That range supports a functional layout rather than one rigid decorative formula.

OATA recommends rockwork, ideally living rock of some description, because it provides natural refuge for marine fish. Fine coral sand is its general substrate recommendation for a marine aquarium. Arrange both around the adult-space decision: a refuge is useful, but the completed structure must not prevent the aquarium from accommodating a fish that can reach 50 cm.

Choose the aquarium style deliberately

FishBase describes the species as a relatively poor swimmer. Design the rock refuge and the usable open space together.

Marine aquaria may be fish-only or include invertebrates such as corals. Water chemistry suitable for a reef does not by itself make every reef inhabitant compatible with this fish. Molluscs, sea urchins, hermit crabs and crabs are all recorded as prey, so the habitat decision and the livestock decision must be made together.

Inspect the completed layout before introduction. Confirm that the rockwork provides refuge, that the substrate suits the intended system, that equipment can operate correctly and that the aquarium still accommodates the planned adult fish. Make structural changes before the animal enters rather than repeatedly rearranging its environment afterwards.

Mature and maintain biological filtration

Use a well set up and maintained biological filter in every aquarium for this fish. The RSPCA’s aquarium welfare guidance explains that fish release toxic ammonia into the water. Beneficial bacteria in a working filter convert ammonia to nitrite, which is also toxic, and then to the safer nitrate.

Prepare marine water with reverse-osmosis (RO) water. OATA recommends RO water for marine systems and considers it essential when invertebrates such as corals are kept. If mixing saltwater at home, dissolve specialised aquarium salt according to its manufacturer’s instructions; do not use table salt. Fast flow from a powerhead and heat from the heater will speed up dissolving the salt. Prepare water for later changes in a separate vessel and bring it to the required conditions before use.

Separate settling from biological readiness

After filling the aquarium and reaching the intended salinity and temperature, switch on the equipment. OATA advises leaving a newly set-up marine aquarium for 24–48 hours so the system can settle, the salt can dissolve fully, the correct temperature can be reached and equipment operation can be checked. Before adding fish, also confirm that ammonia and nitrite are both zero; the equipment settling period is only one part of readiness.

Use measurements, not appearance

• Measure salinity with a hydrometer or refractometer.
• Use test kits regularly to check water quality.
• Confirm ammonia at zero mg per litre before introduction.
• Confirm nitrite at zero mg per litre before introduction.
• Keep nitrate within the limit for the selected marine system.
• Check temperature and pH against the intended range rather than judging water by sight.

Add livestock slowly. OATA explains that the population of beneficial bacteria must increase when fish numbers and feeding increase. Stocking too quickly can leave too few nitrifying bacteria to process the added waste and may lead to new tank syndrome.

Continue maintaining the filter after the aquarium has matured. The biological process depends on a functioning filter and an established bacterial population, so a system that once tested correctly still needs regular testing and maintenance. Any rise above zero ammonia or nitrite is a reason to pause further livestock additions and restore suitable water quality.

Hold stable marine water values

For a fish-only aquarium, keep the water at 24–27°C, pH 8.0–8.2 and salinity of 1.017–1.023 SG, equivalent to 23–30 ppt. Ammonia and nitrite must remain at zero mg per litre, while nitrate must not exceed 20 mg per litre.

For a marine aquarium containing invertebrates, use 24–26°C, pH 8.0–8.2 and salinity of 1.024–1.026 SG, equivalent to 32–35 ppt. Keep ammonia and nitrite at zero mg per litre. OATA gives nitrate at 5–10 mg per litre depending on species, carbonate hardness at 8–12 dKH and phosphate at 0.04–0.1 mg per litre depending on species. Hard-coral systems also require monitoring of parameters such as calcium and magnesium.

Choose one coherent range for the actual aquarium rather than moving casually between fish-only and reef values. The intended inhabitants decide which range applies. If invertebrates form part of the plan, their individual requirements and their compatibility with the porcupinefish both need to pass screening.

Test before introduction and continue testing after livestock is added. If ammonia or nitrite is detectable, delay any new addition. If temperature, pH or salinity is outside the chosen range, restore stable conditions before introducing the fish. Use the hydrometer or refractometer for salinity and suitable test kits for the remaining values.

Feed according to its recorded prey

Base the diet on marine foods that match the species’ recorded animal prey. FishBase records Longspined porcupinefish feeding on molluscs, sea urchins, hermit crabs and crabs at night in nature. Foods based on suitable mollusc and crustacean material therefore provide the clearest basis for an aquarium menu.

Use the natural diet to make three decisions

• Choose the main food by prey type: prioritise marine foods based on molluscs and crustaceans.
• Treat listed prey as a compatibility risk: do not place molluscs, sea urchins, hermit crabs or crabs into the same aquarium on the assumption that they will be ignored.
• Connect feeding to filtration: test water regularly because feeding increases the load that the biological filter must process.

In the wild it feeds at night. Use direct observation of the individual’s activity and food acceptance when organising aquarium feeding, while keeping the food itself aligned with the recorded prey. Set feeding times and amounts by watching how this individual feeds and by keeping ammonia and nitrite at zero.

Make dietary variety relevant rather than random. Different marine foods can contribute variety when they remain based on the mollusc and crustacean prey types recorded for this species. Keep the purpose of each item clear: it should help reproduce the kind of animal material the fish consumes, not simply add another format to the menu.

Track feeding together with water quality. OATA explains that increased feeding requires the filter’s beneficial-bacteria population to increase, just as adding more fish does. A feeding change should therefore be accompanied by regular checks of ammonia, nitrite and nitrate. Ammonia and nitrite must remain at zero, while nitrate must stay within the limit for the aquarium type.

When assessing a proposed food, start with one question: does its main content match the recorded prey of molluscs, sea urchins, hermit crabs and crabs? Then consider whether the fish accepts it and whether the aquarium continues to hold its target water values. This keeps the feeding decision tied to both natural biology and the capacity of the maintained system.

Add livestock and handle arrival carefully

Confirm compatibility and water quality before the fish enters the aquarium. Ammonia and nitrite should both read zero, and temperature, pH and salinity should match the range chosen for the system. Increase livestock numbers slowly so the biological filter can adjust as fish numbers and feeding rise.

Prepare before the parcel arrives

• Have the mature marine aquarium operating at the intended temperature, pH and salinity.
• Test ammonia and nitrite and confirm both are zero mg per litre.
• Check that the aquarium can accommodate the species at its recorded 50 cm adult length.
• Complete the compatibility assessment for every existing inhabitant.
• Arrange for someone to receive the parcel in person.

Follow the arrival sequence

1. Start filming before the parcel is opened, with the sealed parcel and outer box visible.
2. Open the parcel in one continuous take, showing the fish still inside its sealed bag.
3. Check the fish’s condition on the same day the parcel arrives.
4. Float the bag for 20–30 minutes before releasing the fish.
5. Do not pour the bag water into the aquarium.

These arrival steps come from the Tropical Fish Co delivery and Live Arrival Guarantee policy. The continuous video from the sealed parcel onward provides the required view if a Live Arrival Guarantee claim is necessary. If anything has gone wrong, contact us the same day with the order number and video.

Use the first period to protect stability

Continue checking the same decisive values after introduction: temperature, pH, salinity, ammonia, nitrite and nitrate. Keep ammonia and nitrite at zero and avoid adding further livestock quickly. The filter’s beneficial-bacteria population needs to respond whenever fish numbers or feeding increase.

Watch how the fish accepts foods matched to its recorded prey, and keep the aquarium plan centred on one animal by default. Do not turn an observation of temporary quietness, colour or grouping into a conclusion about sex, age or long-term compatibility. Use measured water values and the fish’s recorded biology to guide each practical decision.

Screen every proposed companion before mixing

Plan one Longspined porcupinefish as the default. FishBase describes the wild species as solitary, while also noting that it sometimes forms groups and that young or sub-adult fish may form small groups. Assess any proposed multi-animal arrangement individually.

OATA explains that exact marine stocking numbers vary with body size, species requirements, water parameters and compatibility. Many marine fish are also territorial, which reduces the number that can share an aquarium. Apply those factors to every named companion and assess all animals at their adult requirements.

Check every proposed pairing for aggression, territorial behaviour and fin nipping. Leave out any companion that fails that check. Matching saltwater chemistry is necessary, but it is only one part of compatibility.

Predation is the clearest reason to leave an animal out. Molluscs, sea urchins, hermit crabs and crabs belong under avoid because FishBase records them as food. A larger specimen, a different arrangement of rockwork or matching chemistry does not remove its place on the recorded prey list.

For a proposed marine-fish companion, check the adult space, water range, territorial behaviour and dietary interaction together. Leave out any proposed companion that does not pass every one of these checks.

Decide between fish-only and reef plans carefully

A fish-only marine plan provides the clearest starting framework for this species. Keep it at 24–27°C, pH 8.0–8.2 and 1.017–1.023 SG, then screen every fish companion for adult size, territorial behaviour and compatibility. The aquarium still needs to accommodate the porcupinefish at its recorded maximum of 50 cm.

A reef aquarium adds a second decision layer because it contains invertebrates. Its chemistry should follow the marine-and-invertebrate range of 24–26°C, pH 8.0–8.2 and 1.024–1.026 SG, with carbonate hardness at 8–12 dKH. However, matching those values does not make an invertebrate safe with the fish.

Place recorded prey under avoid

• Molluscs
• Sea urchins
• Hermit crabs
• Crabs

FishBase records all four as natural food. Avoid housing them with the porcupinefish rather than treating them as possible companions after routine screening. Their role as recorded prey is a direct biological conflict, not merely a difference in preferred water values.

Treat the rest of a reef plan as undecided

Assess every other intended invertebrate individually before using a reef label for the complete aquarium. Add any other invertebrate only after assessing it individually against this fish’s diet and behaviour. Leave out any invertebrate that does not pass that assessment.

Water choice must follow the complete livestock plan. If the system contains invertebrates, use the marine-and-invertebrate values and monitor nitrate, carbonate hardness and phosphate as OATA directs. If it is fish-only, use the fish-only values. In either case, keep ammonia and nitrite at zero and increase stocking slowly so biological filtration can adjust.

Answer common keeper questions

How big does the Longspined porcupinefish get?

FishBase records a maximum of 50 cm total length. Plan the aquarium around that full adult figure rather than around the fish’s size on arrival.

What size aquarium does it need?

For this 50 cm marine fish, choose an aquarium that accommodates it fully grown after rockwork and substrate are installed; assess usable interior space, filtration and every inhabitant’s adult requirements rather than relying on a litre figure alone.

How many Longspined porcupinefish should I keep?

Plan one animal by default. FishBase describes the wild species as solitary, although it sometimes forms groups and young or sub-adult fish may form small groups, so any multi-animal aquarium needs an individual assessment.

Is the Longspined porcupinefish suitable for beginners?

Choose it only if you can plan for a 50 cm adult, maintain stable marine water and mature biological filtration, provide prey-matched food and screen every companion carefully; treat it as a substantial large-marine-fish commitment.

What does it eat?

FishBase records this fish eating molluscs, sea urchins, hermit crabs and crabs at night in nature. Base its aquarium diet on suitable marine foods made from mollusc and crustacean material.

What temperature and pH does Diodon holocanthus need?

OATA’s marine fish-only guidance gives 24–27°C, pH 8.0–8.2 and salinity of 1.017–1.023 SG; keep ammonia and nitrite at zero mg per litre and nitrate at no more than 20 mg per litre.

Is it aggressive or likely to nip fins?

FishBase describes this fish as solitary and a relatively poor swimmer, with young or sub-adult animals sometimes forming small groups. Screen each proposed pairing for territorial behaviour, aggression and fin risk rather than assuming a universal temperament.

Which animals can the Longspined porcupinefish live with?

Avoid molluscs, sea urchins, hermit crabs and crabs because FishBase records them as prey. Consider other marine fish only after checking shared water values, adult space, territorial behaviour and dietary interaction.

How does it arrive alive in the UK?

Tropical Fish Co packs live fish in pure oxygen inside an insulated poly box, using a heat pack when the weather calls for it, and sends tracking for the licensed live-animal courier; someone must be present to receive the parcel, must film continuously from the sealed parcel through the fish’s condition in its sealed bag, and must open and inspect the parcel the same day it arrives.

Consult the public references

• FishBase — Diodon holocanthus: identity, family, distribution, habitat, behaviour, natural diet, maximum recorded length and conservation classification.
• Wikipedia — Long-spine porcupinefish: species overview and established alternative common names.
• OATA — How to set up and look after a marine aquarium: saltwater preparation, aquarium setup, routine marine values, testing, stocking and acclimatisation guidance.
• RSPCA — Fish environment and filtration: biological filtration and aquarium waste processing.