Fat sleeper

1) Where they come from

Fat sleepers (Dormitator maculatus) are a coastal fish from the western Atlantic—Florida and the Gulf down through the Caribbean and parts of Central America. You’ll find them in the messy “in-between” places: mangroves, river mouths, ditches, and lagoons where the salinity swings around.

That background explains a lot: they’re tough, they like structure, they eat like little trash compactors, and they’re fine with brackish water that isn’t perfectly stable—though your tankmates might not be.

2) Setting up their tank

Give them space and a layout that breaks line-of-sight. A fat sleeper looks kind of chill until it decides a whole corner is “its” corner. The bigger the fish gets, the more you’ll appreciate extra footprint.

• Tank size: 40 breeder minimum for a single adult, 75+ if you want tankmates (they’re bulky and territorial).
• Substrate: sand or smooth fine gravel. They spend a lot of time on the bottom and like to shuffle around.
• Hardscape: rocks, driftwood, and piles of rubble to make caves and barriers. I use 3–4 distinct hide zones so nobody has to share.
• Filtration: heavy. Think “messy predator.” Strong bio filtration plus a good prefilter sponge helps a lot.
• Flow: moderate. They don’t need a river tank, but dead spots become gross fast with these guys.

Salinity-wise, I’ve had the best long-term results in low to mid brackish—roughly SG 1.005–1.012 depending on what else you keep. Temperature in the mid-70s to around 80°F works well, and they’re not picky about pH as long as it isn’t crashing from neglected maintenance.

3) What to feed them

They’re enthusiastic eaters. Mine would act like it hadn’t eaten in weeks even when it was clearly built like a potato with fins. The trick is feeding them well without turning your tank into a nitrate factory.

• Staples: quality sinking carnivore pellets (they learn fast) and frozen foods like mysis, krill, chopped shrimp, clam, and bloodworms.
• Treats: live blackworms (if you can get them clean), ghost shrimp, small crabs from a safe source.
• Vegetable matter: not required, but occasional spirulina-based foods can help round out the diet.

Feeding schedule: juveniles can handle smaller daily meals, but adults do better with every-other-day or smaller portions once a day. If the belly is staying rounded all the time, you’re overdoing it.

4) Behavior and tankmates

They’re ambush predators with a laid-back vibe… until something fits in their mouth. Most of the day they’ll sit, watch, and reposition like they’re plotting. At feeding time they turn into a vacuum cleaner.

Tankmates are all about size, attitude, and where they hang out. Bottom-dwellers get the most trouble because that’s the sleeper’s real estate.

• Good bets (with enough space): larger monos, scats, archerfish, knight gobies, larger brackish cichlids (depending on your salinity), tough midwater fish that won’t be bullied off food.
• Risky: other bottom fish (especially gobies you actually like), slow fancy fish, anything slender that can be swallowed.
• Definitely not: small community fish, tiny mollies/guppies unless you’re okay with them becoming snacks.

5) Breeding tips

Breeding them in home aquariums isn’t common. They can spawn in brackish-to-fresh situations in the wild, and the early life stages are the hard part—larvae often want very specific conditions and foods that most of us aren’t set up for.

If you want to take a swing at it, think in the direction of a dedicated breeding setup: a well-fed conditioned pair, lots of caves/structures, and a plan for tiny live foods (rotifers, baby brine, etc.). I’d treat it like a “project fish,” not a casual bonus spawn.

6) Common problems to watch for

• Overfeeding and dirty water: the #1 issue. They eat big and poop big. If nitrates climb, they get sluggish and more prone to disease.
• Mouth injuries: they’ll lunge at hard prey or smack into decor during feeding frenzies. Watch for swelling or fuzzy patches.
• Skin/fin infections after fights: torn fins can get nasty quickly in warm brackish tanks.
• Ich/velvet confusion: brackish fish can still get parasites, and salt levels that are “kind of brackish” don’t magically prevent it.
• Salinity mismatch stress: moving a fish from fresh to brackish (or the other way) too quickly can wreck them.

The best “medicine” for fat sleepers is boring consistency: big water changes, strong filtration, and not letting leftover food rot in the sand. If you keep the tank clean and don’t try to house them like a community fish, they’re surprisingly hardy for an advanced brackish predator.