Jack Dempsey Cichlid UK: Care, Tank Size & Honest Buying Guide

Why this guide exists

You searched "Jack Dempsey", which usually means one of two things: either a glittering blue-green adult caught your eye in a tank, or you've already bought a tiny one and you're starting to suspect it's going to get bigger — and stroppier — than the shop let on. Both are good reasons to read on.

I'm Kevin, the editor and owner here, and cichlids have been one of my keeping obsessions for the best part of fifteen years. The Jack Dempsey is a fish I love and a fish I'm careful about recommending, because the two things that decide whether you'll enjoy one — the real adult tank size and the temperament — are exactly the two things a quick shop chat tends to gloss over. This is the honest guide I'd give a friend who messaged me saying "I really want a Jack Dempsey — talk me into or out of it."

A standard Jack Dempsey in our holding tank. Those blue-green spangles aren't fixed pigment — they're structural colour that intensifies as the fish settles and gains confidence, which is why a happy Dempsey looks like a completely different animal to a stressed one. Product photo · our warehouse.

Five things most UK guides never tell you

• It changed genus in 2007. For decades this fish was Cichlasoma octofasciatum, but a peer-reviewed revision by Schmitter-Soto moved it into a brand-new genus, Rocio — so the correct modern name is Rocio octofasciata ^[2]. You'll still see the old name on shop labels (ours included, until the supplier catalogue catches up), but they're the same fish.
• The name is a boxing reference. "Jack Dempsey" honours the 1920s world-heavyweight champion of the same name — a nod to the fish's hard-hitting, in-your-face temperament and its scowling "face" ^[3]. It is one of the few aquarium fish named after a human athlete.
• The blue spangles are a mood ring. The iridescent flecks across the body intensify when the fish is confident and settled and fade when it's stressed, cold or freshly moved ^[3]. If your Dempsey goes drab, read it as a warning light, not a defect.
• Escapees survive almost anywhere warm. The US Geological Survey records feral Jack Dempsey populations established from aquarium releases in several states ^[4]. That's a vivid illustration of just how hardy and adaptable this fish is — and exactly why you must never, ever release an unwanted one into UK waters.
• It's a Central American fish, not South American. Its natural range runs from southern Mexico down through Belize, Guatemala and Honduras ^[1] — slow, warm, often murky lowland waters. That heritage is why it shrugs off hard water and a wide temperature band.

The honest comparison: is a Jack Dempsey right for you?

Before the care detail, the decision. A Jack Dempsey sits in the middle of the New World cichlid range — bigger and feistier than the dwarf cichlids, smaller and (slightly) more manageable than an Oscar or a Flowerhorn. Here's how it stacks up against the usual alternatives keepers cross-shop:

If you want a serious cichlid but can't house an Oscar, the Jack Dempsey is the sweet spot — provided you respect the ~200 L floor ^[1]. If the aggression worries you, the Electric Blue morph is the gentler way in. If you want a community-tolerant centrepiece instead, read our Firemouth guide.

The Jack Dempseys we currently stock

These are the two genuine Jack Dempseys in stock right now — the classic standard fish and the line-bred Electric Blue morph. Everything else lower down this page is a tank-mate or alternative big cichlid, clearly labelled as such; these two are the fish this page is actually about.

The standard fish is the bargain entry point and the tougher of the two; the Electric Blue is the showpiece and the gentler temperament, but it's a more delicate animal that wants more stable water. Pick based on how much fight you want in your tank, not just on looks.

The Electric Blue Jack Dempsey: a different beast

The Electric Blue Jack Dempsey (EBJD) deserves its own section because it's so widely misunderstood. It is not a separate species and it is not a wild fish — it's a line-bred recessive colour morph of Rocio octofasciata, fixed by selective breeding so the whole body glows electric blue rather than carrying spangles on a dark base ^[3].

Three things follow from that recessive genetics:

• It's smaller. EBJDs typically top out a little under the standard fish, around 20 cm, and grow more slowly ^[3].
• It's more delicate. The same line-breeding that fixes the colour also makes EBJDs more sensitive to poor water and less vigorous than standard fish — they're an intermediate keeper's fish, not a beginner's.
• It's noticeably less aggressive. Most keepers find EBJDs calmer than standard Dempseys, which makes a robust, well-planned community a little more feasible — though it's still a predator, so the tank-mate rules below still apply.

How big a tank, really — and how many fish

Let's nail the numbers, because this is where Jack Dempsey keeping succeeds or fails. The species reaches about 25 cm ^[1], and it's a heavy-bodied, bottom-patrolling fish that claims territory.

• One fish: about 200 litres, ideally a 120 × 45 cm footprint so it has length to patrol. This is the easiest, lowest-drama setup there is.
• A bonded pair: 250–300 litres or more, with the décor broken up so the subordinate fish always has somewhere out of sight to retreat ^[3].
• Don't keep two unbonded adults, and especially not two males — they'll fight, often badly.

Unlike shoaling fish, you don't "buy a group" of Jack Dempseys. The right number is one, or a genuine male/female pair if you intend to breed. Everything you add beyond that is a tank-mate decision, not a stocking-up of the same species.

Tank mates: choose robust, or keep it solo

Here's the honest rule: a Jack Dempsey is a predator that will eat anything small enough to fit in its mouth — tetras, guppies, dwarf shrimp, the lot ^[3]. It's also territorial, so even similarly-sized fish can get harassed. Plenty of experienced keepers conclude that the simplest, kindest setup is a solo fish or a single bonded pair, full stop ^[5].

If you do want company, the formula is: a big tank, robust similarly-sized cichlids, and an armoured bottom-dweller that can look after itself. Good candidates to consider — in a tank of 400 litres or more, with plenty of broken sightlines:

• Other large, robust Central/South American cichlids of similar size and attitude (see the in-stock block below).
• Larger plecos — armoured, nocturnal and able to shrug off a territorial cichlid; a classic Dempsey tank-mate that occupies a different zone.
• Robust corydoras-style catfish can work in bigger setups, though anything truly small is a snack — match the catfish to the tank, not the other way round.

Avoid: anything small or peaceful, long-finned fish (fin targets), and a second Jack Dempsey unless it's a true bonded pair.

Watch: a community tank in motion

Transcript / what you're seeing (0:00–0:20): a planted tank with small fish cruising and shoaling through the mid-water and upper layers, calm and spread out. A Jack Dempsey tank looks nothing like this — instead of a peaceful crowd, you get one (or a pair of) large, deliberate fish patrolling the bottom third, claiming a cave or a piece of wood as territory. If you're imagining a Dempsey gliding peacefully among tetras like the clip above, that's exactly the mismatch this guide is trying to head off: those tetras would be lunch.

What they eat (and what not to feed)

Jack Dempseys are carnivores and opportunistic predators ^[1], so feed accordingly — but don't overdo it, because they pile on weight and pollute heavily:

• Staple: a good-quality sinking carnivore/cichlid pellet sized to the fish.
• Frozen treats: bloodworm, brine shrimp, mussel, prawn and the occasional earthworm for condition and colour.
• Vegetable matter: a small amount of plant-based food rounds out the diet, even for a carnivore.
• Skip the feeder fish. Live feeders are an unnecessary disease and parasite risk, and a varied frozen/pellet diet grows a healthier fish ^[3].

Feed once or twice a day, only what's eaten in a couple of minutes, and run more filtration than the tank "needs" — a well-fed Dempsey is a messy fish.

A glimpse of the Electric Blue morph

The Electric Blue Jack Dempsey (SKU S068). Compare it with the standard fish in the hero shot: the morph replaces the dark base and scattered spangles with an all-over electric-blue sheen. It's the same species — just a recessive colour line that's smaller, gentler and more delicate. Product photo · our warehouse.

Sexing and breeding

Sexing adults isn't hard: males grow larger, develop longer, more pointed dorsal and anal fins, and tend to be more strongly coloured; females are rounder and often show a darker patch on the flank and gill cover ^[3]. They're substrate-spawners — a bonded pair will clean a flat rock or pit and lay eggs on it, then guard the eggs and fry together with the ferocity the name promises.

That parental aggression is exactly why breeding needs space and planning: a spawning pair will try to clear the entire tank of every other fish ^[5]. If you're not deliberately breeding, a solo fish sidesteps all of this. If you are, give the pair their own large tank and be ready to separate them if the bond breaks down.

What to look for when you buy (anywhere)

Whether you buy from us or anyone else, these are the welfare markers that separate a healthy Jack Dempsey from a problem one:

• Good colour and confidence. A settled fish shows its spangles and holds its fins up. A persistently drab, clamped, hiding fish may be stressed or unwell ^[3].
• Clear eyes, clean skin, intact fins. No white fuzz, no ulcers, no "pop-eye", no split or ragged fins.
• A rounded but not bloated belly, and no stringy white waste trailing the fish (a parasite warning).
• Active and responsive to movement at the glass — a predator should be alert, not listless.
• Ask the species and the morph. A seller who can tell you it's Rocio octofasciata (and whether it's a standard fish or the Electric Blue morph) is a seller who knows their stock ^[2].

Community & clubs

Big New World cichlids reward you for learning from people who keep them, and the UK has a dedicated society for exactly this:

• The British Cichlid Association (BCA) is the UK club for cichlid keepers — talks, shows, a members' journal and a network of breeders who keep everything from dwarf species to big Central Americans. It's the single best place to learn Jack Dempsey keeping properly and to meet people who've raised pairs (britishcichlid.org/about).
• Seriously Fish maintains the most reliable independent species profile for cross-checking care details on Rocio octofasciata ^[3].

When your Jack Dempsey arrives: acclimation

Jack Dempseys are hardy and tolerant of hard UK water ^[4], so they don't need the fussy, ultra-slow drip a sensitive soft-water fish demands. The priorities are temperature, calm, and immediate cover — a big, territorial fish settles fastest when it can claim a hiding spot straight away:

1. Float the sealed bag in the tank for 15–20 minutes to equalise temperature.
2. Drip or top-up acclimate over 20–30 minutes, roughly doubling the bag volume with tank water so it adjusts to your pH and hardness gradually.
3. Net the fish into the tank — don't tip the transport water in.
4. Have caves and wood in place on day one. Immediate territory and broken sightlines dramatically cut a cichlid's transport stress ^[5].
5. Lights off and no feeding for the first 24 hours. Let it find its territory and calm down before its first meal; don't be alarmed if it looks pale and hides at first — the colour comes back as it settles ^[3].

Ready for more?

• Learn: our cichlid care guide for the fundamentals of keeping New World cichlids, the pleco care guide for the armoured tank-mate that pairs best with a Dempsey, and the corydoras care guide for matching catfish to the right tank.
• Compare: the Oscar guide if you've got room for something bigger, the Firemouth guide for a more community-tolerant centrepiece, or the Flowerhorn guide for the hybrid powerhouse — and the German Blue Ram guide if you actually wanted the tiny "electric blue" fish, not this one.
• Shop: browse the South American & New World cichlid hub for everything in the family, or the wider tropical fish for sale hub for the full UK range.