Tanganyika Clown Goby (Eretmodus cyanostictus): Care for a Proper Oddball

If you want a Tanganyikan that behaves nothing like the rest of the lake’s cichlids, the clown goby is it. Eretmodus cyanostictus does not glide; it hops and props itself along the rockwork like a little gargoyle, grazing algae off the stone and watching you with a face that always looks mildly affronted. It is one of the few fish I would call genuinely characterful rather than just pretty. It also happens to be a true hard-water specialist, which makes it a natural fit for the kind of chalky, alkaline tap this site is built around. The catch is that it asks for two things a lot of keepers get wrong: a lot of oxygen, and a diet with almost no meat in it.

What you are actually keeping

Eretmodus comes from the surge zone of Lake Tanganyika, the shallow band right where the waves break over the rocks. That single fact explains almost everything about the fish. Its swim bladder is reduced to the point of being more or less useless, which is why it sinks and hops rather than hovers; staying planted against moving water is the whole point. Adults top out around 8cm, so this is not a big fish, but it carries itself like one.

The body is a dark olive to grey, broken by rows of pale blue spots and faint vertical bars, and the spotting is where the “cyanostictus” name comes from. There are several regional variants in the trade, labelled by collection point, and they are broadly similar to keep. What matters more than the variant is that you buy healthy stock, because like a lot of slow-growing Tanganyikans these are easy to stunt or lose if they arrive underfed.

Water: the easy part on hard tap

Here is where my water does the work for me. Eretmodus wants it hard and alkaline, roughly 10 to 25 dGH and a pH of 8.0 to 9.0, with a temperature of 24 to 28°C. My tap runs at about 22.5 dGH, so the hardness is handled before I even fill the tank. If anything the fish sits at the comfortable end of its range without any effort on my part.

The one number I do keep an eye on is pH. Straight from my tap the water sits around 7.2, which is a touch low for a fish that likes it genuinely alkaline, so I lean on the carbonate buffering that Rift Lake tanks need anyway. An aragonite or coral-based substrate, or a bag of crushed coral in the filter, holds the pH up in the 8s and keeps it stable through the week. If that side of things is new to you, my guide on buffering hard water for Rift Lake tanks covers how I do it, and GH and KH explained sorts out which number is doing what. Soft, acidic water is the one setup where I would tell you to pass on this fish entirely; it is the wrong lake for it.

The book behind Lagonia

The Hard Water Aquarium

My practical guide to working with hard, alkaline tap water instead of fighting it: reading your water, choosing and setting up the tank, and keeping and breeding the fish that thrive in it, from livebearers to Rift Lake cichlids and shell-dwellers.

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Oxygen is not optional

This is the part people underestimate, and it is the part that kills clown gobies. A surge-zone fish is used to water that is churning and saturated with oxygen every second of the day. In the tank that means serious flow and serious surface movement, not a gentle trickle.

I aim for a turnover of about four to five times the tank volume an hour, and I point the return so it ruffles the surface hard rather than firing a jet across the glass. Broken surface is what actually gets oxygen into the water, so I would rather have visible ripple than a clean, glassy top. In a warm tank this matters even more, because warm water holds less dissolved oxygen just as the fish’s metabolism is asking for more of it. If you are not sure your tank is moving enough air, an extra sponge filter run off a pump, or a simple airstone, is cheap insurance; I go into when it earns its place in do you need an air pump, and choosing an aquarium filter covers getting the turnover right in the first place.

Feeding: keep the meat out

Eretmodus is a grazer. In the lake it scrapes a living off the algae and tiny organisms coating the rocks, which we call aufwuchs, and its gut is built for that green, fibrous diet. Feed it like a carnivore and you are asking for trouble, specifically the digestive blowout that Rift Lake keepers call bloat.

So the base of the diet is vegetable: a good spirulina flake or pellet, blanched spinach or deshelled peas, and whatever algae it can rasp off the rockwork itself. I keep the lighting fairly bright over at least some of the stones precisely so a green film grows for them to work at; it keeps them busy and it keeps them fed the way they are meant to be. Small frozen foods like cyclops are fine as an occasional extra, but I steer well clear of the rich stuff, no bloodworm, no beefheart, no shellfish. If a fish of mine ever does start sulking with stringy white waste trailing behind it, I treat it as early bloat straight away; my write-up on African cichlid bloat applies just as much to the Tanganyikans, and the same diet that causes it in Tropheus causes it here. On that note, if you have kept Tropheus you already understand this fish’s stomach; they are the two species I am fussiest about feeding.

Temperament and tankmates

Clown gobies are territorial towards their own kind and faintly grumpy in general, but they are not big aggressive brutes. The usual advice, and the advice I follow, is to keep them either singly or as a single bonded pair. A pair that forms tends to stay together for life, which is a lovely thing to watch, but getting there is the hard part; you really want to buy a small group of youngsters, let them sort themselves out, and then move the spare fish on once a pair is obvious. Trying to run several pairs in one tank only works if that tank is genuinely large, because the males will not share ground.

For tankmates, think about niches rather than just temperament. Eretmodus owns the lower rockwork, so the fish that work best are the ones living somewhere else in the tank. Open-water schoolers like Cyprichromis leptosoma are close to ideal; they hang in the upper water and leave the gobies alone. Shell-dwellers on a sand patch to one side also coexist well, since they are busy with their own little snail-shell kingdoms down in the open substrate. What I would not do is put clown gobies with boisterous Malawi mbuna or anything fast and food-greedy; the gobies are slow eaters and will simply be out-muscled at every meal. Whatever you mix, run the numbers before you add fish, because a rocky Tanganyikan tank fills up faster than it looks. My stocking calculator and the tank volume calculator will keep you honest, and how many fish can I keep explains why the box litres lie.

Tank size and setup

For a single fish or a settled pair I would not go below a tank of around 90cm by 30cm, which is roughly 80 litres of real water, and more length is always better because it gives a second fish somewhere to retreat. The aquascape is simple and it plays straight to the fish’s niche: a sand base, and a pile of rounded rock worked into caves and ledges so there is plenty of surface to graze and plenty of line-of-sight breaks to defuse squabbles. My notes on hardscape with rock and wood and substrate for Tanganyikan tanks both apply here; stable rockwork sitting on the glass, not on the sand, is the only safety rule I am strict about.

Plants are optional and mostly for your benefit rather than theirs. If you want them, the tough hard-water survivors like anubias and java fern tied to the rock are the sensible choice, since the gobies will ignore them and the hard water will not. I run through the realistic options in hard-water aquarium plants.

Breeding: the shared brood

This is the detail that makes Eretmodus special, and it is rare enough in cichlids to be worth the effort of a dedicated pair. They are biparental mouthbrooders, which means both parents take a turn carrying the eggs. The female lays only one or two eggs at a time and picks them straight back up, ending with a small clutch of maybe ten to thirty fertilised eggs in her mouth. She holds for a couple of weeks, and then, remarkably, she passes the whole brood across to the male, who finishes incubating them for another week or two before releasing free-swimming fry.

That handover is genuinely unusual. Monogamy paired with shared mouthbrooding duty barely exists anywhere else in cichlids, and watching a pair you raised pull it off is one of the quiet highlights of keeping Tanganyikans. Do not expect post-release babysitting, though; once the fry are out, they are on their own, and in a community tank most will be eaten. If you want to raise them, the fry take baby brine shrimp and powdered spirulina from day one, so a separate grow-out tank is the way to actually keep a few. A heater sized properly for that small tank helps keep growth steady; my heater calculator will tell you the wattage.

Is it the fish for you?

I would steer a complete beginner towards something more forgiving, not because the clown goby is fragile once settled but because its two demands, high oxygen and a strict vegetable diet, are the exact things new keepers tend to get wrong. Give it a mature tank with real flow, keep the meat out of its food, and let it sit on hard alkaline water that it would have to fight for anywhere else, and you get a slow-growing, long-lived little character that does something almost nothing else in the hobby does. For anyone already running a Tanganyikan setup, it is one of the most rewarding oddballs the lake has to offer. If you are still putting that tank together, start with my full Tanganyika tank setup guide.