Best Fish for a 30 Litre Tank: Honest UK Guide

Direct answer: there is no responsible universal number of fish for a 30 litre tank. The clearest fish route in the selected UK guidance is one male Betta, because OATA recommends at least 20 litres for one male. However, the RSPCA recommends choosing an aquarium larger than about 40 litres. A nominal 30 litres therefore sits at a restrictive boundary—not at the start of a miniature community.

If the exact tank cannot provide suitable inner dimensions, usable water, mature filtration, stable heat, gentle flow, cover and tested water for one male Betta, choose a larger aquarium. A species-focused shrimp or snail aquarium is a separate alternative. Do not combine fish and invertebrates automatically, and do not use a fish-per-litre formula.

A nominal 30 litres is about 7.9 US gallons or 6.6 imperial gallons, using NIST conversion factors^[1]. Substrate, rocks, wood, equipment and the air gap reduce the water actually available. Two tanks carrying the same 30 L label can also provide very different swimming length, floor area and gas exchange.

Commercial disclosure: Tropical Fish Co sells live animals. This article owns the planning decision; the exact identity, sale unit, size, price and availability belong on the current product page. A pictured or discussed animal is a research candidate—not a stock claim, basket recipe or compatibility guarantee.

Why the old “how many fish” answer was removed

The former page gave fixed recipes such as six Ember Tetras, six to eight chilli rasboras, eight Endlers, six Sparkling Gouramis or one Betta plus shrimp and snails. It also claimed a maximum of 12–15 small fish. Those statements made litres do work they cannot do.

A simple count ignores:

• adult body size and mass;
• swimming activity and the tank's actual length;
• social-group and territorial behaviour;
• surface area, oxygen and filter maturity;
• measured pH, hardness and temperature;
• sex, reproduction and fry;
• every existing resident; and
• the difference between gross capacity and usable water.

The RSPCA says larger volumes provide more stable conditions, recommends choosing a tank larger than 45 × 30 × 30 cm—about 40 litres—and treats fish-length calculations only as guidelines because temperature, source water, filtration and maintenance also affect water quality^[2]. OATA likewise requires exact-species research, a mature filter, zero ammonia and nitrite and gradual additions^[4].

Google's result page currently answers this query with an AI-generated fixed number. That search feature was used to understand the unanswered question, not as a husbandry source. The recommendations below come from the cited welfare, government, trade-body and scientific records.

Measure the tank before choosing any animal

Measure the aquarium's inside rather than relying on a model name. For a rectangular tank:

inner length (cm) × inner width (cm) × filled water depth (cm) ÷ 1,000 = approximate litres before displacement

Then record:

1. inner length and width;
2. approximate usable water after substrate and décor;
3. surface access and cover;
4. stable temperature measured with an independent thermometer;
5. pH, general hardness (GH) and carbonate hardness (KH);
6. ammonia, nitrite and nitrate trend;
7. the exact filter and its flow; and
8. every existing animal at adult size.

UK tap water is not one chemistry. The Drinking Water Inspectorate says hardness varies with geology and local supply and directs readers to the water company for an accurate local result^[5]. Use that report as context, then test the aquarium and replacement water. “UK tap water” is not a species-suitability claim.

Use the tank-volume calculator for the estimate, compare measured water with the water matcher, and screen current residents with the tank-mate tool. These tools organise evidence; they cannot guarantee capacity or compatibility.

Route 1: one male Betta, conditionally

One male Betta splendens is the clearest fish candidate—not permission for a Betta community. OATA recommends at least 20 litres for one male, compared with at least 60 litres for a female group. It also requires filtration, heating, gentle flow, cover, planting or suitable shelter and access to air at the surface^[6]. FishBase supports the current species identity but does not set a home-aquarium stocking recipe^[7].

A 30 litre Betta plan must pass all of these checks:

• the usable water remains above the relevant source boundary after displacement;
• the footprint lets the individual swim and turn without sharp décor;
• a mature filter keeps ammonia and nitrite at zero without excessive current;
• tested pH, GH, KH and temperature fit the animal and the seller's holding-water transition;
• the aquarium is covered while leaving warm air and surface access;
• planting or smooth shelter provides cover without obstructing the surface; and
• a separate mature enclosure is ready if any proposed companion fails.

Do not add a second male. Do not treat a female group as an alternative at this size. OATA warns that mixing Bettas with other fish can be difficult, and the RSPCA notes that male Bettas may fight with their own kind while shoaling species need proper groups^[3].

The RSPCA's broader recommendation remains more cautious than the OATA species sheet. This page does not hide that difference: 30 litres may pass OATA's single-male volume screen, but a larger aquarium can offer more stable water and greater design margin.

Route 2: a species-focused shrimp or snail aquarium

Shrimp and snails are not fish, but a carefully planned invertebrate aquarium can make better use of a small footprint than an undersized mixed fish community. They are animals with complete needs—not equipment for removing algae or leftovers.

OATA recommends at least 10 litres for a group of smaller freshwater shrimp or snails and about 20 litres for larger examples such as Amano Shrimp. It also says the aquarium must be larger when they are kept with fish^[12]. Its group-level sheet cannot identify the exact animal in a shop, so verify species or trade-form identity and use the exact care guide.

Before choosing this route:

• mature the filter and confirm zero ammonia and nitrite;
• match temperature, pH, GH and KH to the exact animal;
• protect the filter intake and provide appropriate cover;
• provide a complete diet instead of relying on visible algae;
• check every medicine or additive for copper and invertebrate safety;
• plan for reproduction before a colony expands; and
• add animals gradually while monitoring water and behaviour.

Continue with the Shrimp Keeping guide, Cherry Shrimp guide, Amano Shrimp guide or Nerite Snail guide. No fish is guaranteed shrimp-safe, and a snail's family-level trade label is not enough to determine adult size, diet or reproduction.

Fish groups that do not pass the 30 litre screen

The old page used small shop size as a substitute for a complete adult plan. Current broad UK guidance gives these starting boundaries before exact species, dimensions, water and social needs are considered:

This table is a shortlist stop, not proof that the stated boundary is ideal. Upgrade to the 60 litre guide and still check the exact internal length, water and adult social plan.

How to answer “how many fish?” without a formula

Use a staged capacity gate:

1. Choose one exact route. One male Betta or one species-focused invertebrate plan—not both by default.
2. Verify identity and adult needs. A common name, shop size or colour does not settle the species.
3. Fit the complete social plan. Do not isolate shoaling fish to make the litres appear sufficient.
4. Finish fishless maturation. Ammonia and nitrite must rise and then return to zero and remain there; elapsed weeks alone do not prove readiness.
5. Check the adult layout. Preserve the surface, cover, open water and floor area the selected animal uses.
6. Add deliberately. Follow the exact social and acclimation plan rather than adding random “starter fish”.
7. Measure again. Record water results, feeding, breathing, swimming, hiding, chasing and injury.
8. Stop when evidence is uncertain. An empty-looking layer is not unused biological capacity.

If an animal hides continually, cannot feed, gasps, chases, is chased or shows injury, test the water immediately and separate when needed. Contact a qualified aquatic vet or knowledgeable retailer for diagnosis. This guide gives no medicine, heat, salt or dosing recipe.

Equipment without a one-size shopping list

A mature filter is essential. Choose flow that supports gas exchange and filtration without preventing the selected animal from swimming normally. Protect the intake where shrimp or small animals could be drawn in.

There is no universal heater wattage for “30 litres”. Heater choice depends on room temperature, target temperature, aquarium shape, cover, circulation and the manufacturer's current rating. Where the selected tropical animal requires heat, use a thermostatically controlled heater and verify stable water temperature with an independent thermometer^[2].

Buy tests for ammonia, nitrite, nitrate trend and pH; measure GH and KH for species selection. OATA says a filter matures when ammonia and nitrite have risen, fallen to zero and remain at zero—not when a fixed four- or six-week calendar expires^[4].

Purchase-ready 30 litre checklist

Do not buy an animal until every line has a defensible answer:

• Gross label, inner dimensions and approximate usable water are recorded.
• The exact animal's adult space and social needs fit the measured footprint.
• pH, GH, KH and temperature are measured rather than guessed.
• The filter is mature and ammonia and nitrite are zero.
• One male Betta or one invertebrate species plan is selected—not a mixed recipe.
• The exact identity and adult size are verified from a reliable source.
• Flow, cover, surface access, substrate and shelter fit that animal.
• Every animal has a complete diet; none is purchased as cleaning equipment.
• Sex, reproduction and a realistic destination for young are planned.
• Every current resident is included in capacity and compatibility checks.
• A mature separation option exists before any companion is attempted.
• Current product identity, sale unit, size and availability are checked on the exact listing.

Read the first tropical tank guide for fishless maturation and the water chemistry guide before choosing livestock. For the fish route, continue to the Betta guide. Only after those checks should you browse freshwater fish. Stock changes; this article contains no product card, SKU, price or availability promise.

How this guide was checked

The Tropical Fish Co Editorial Team rebuilt this page on 6 September 2026. Unit conversions come from NIST; aquarium welfare and the broader over-40-litre recommendation come from the RSPCA; setup and species-group boundaries come from OATA; UK hardness context comes from the Drinking Water Inspectorate; and FishBase is used narrowly for current Betta identity.

Owned UK-mobile Search Console evidence showed that “how many fish” questions were the page's largest unanswered opportunity. A current external mobile result also showed Tropical Fish Co being cited in Google's AI Overview for the old fixed stocking recipes. Search results, competitors, forums and AI summaries were used to understand intent and misinformation—not as husbandry authorities.

The old universal under-4-cm rule, 12–15-fish maximum, eight fixed stocking recipes, “clean-up crew”, UK-tap-water promise, fixed equipment prices, universal heater wattage and calendar-only cycling claim were removed because the selected evidence did not support them.

If you spot a source conflict, use the correction box below. A correction request is editorial feedback—not a lead, sale or ecommerce conversion. Purchases and revenue are the final ecommerce outcomes. Add-to-cart and begin-checkout are diagnostic funnel signals, not leads or final conversions; forms, email, feedback, newsletters, back-in-stock requests and generic events are never leads or ecommerce conversions.