Similis annual killifish

Where they come from

Similis annual killifish (Simpsonichthys similis) are one of those fish that make you realize nature is a bit wild. They come from seasonal pools in parts of Brazil where water shows up, heats up, gets crowded with life... and then disappears. The fish race the clock: grow fast, breed fast, and their eggs ride out the dry season in the mud until the rains return.

That background explains almost everything about how they act in a tank: quick to mature, very food-driven, not big on long-term stability if you treat them like a typical community fish.

Setting up their tank

Think small, simple, and easy to keep clean. I like a species tank for these. They are not big fish, but they live fast and they appreciate fresh water and predictable routines.

• Tank size: 10-20 gallons works well for a trio or small group. Bigger is fine if you keep them as the main focus.
• Filtration: gentle sponge filter or a small internal filter turned down. You want clean water without blasting them around.
• Substrate: bare bottom is easiest for maintenance, but if you plan to breed, you will add a spawning medium anyway (see Breeding).
• Cover: a tight lid. Killifish jump. They do not warn you first.
• Light: moderate. Too bright and they can stay skittish, especially in a bare tank.
• Plants: floating plants help a lot (frogbit, salvinia). They break up sight lines and calm the tank down.

Water-wise, aim for stable, clean freshwater. Slightly soft to neutral is usually the easy lane with Simpsonichthys, but the bigger deal is keeping nitrates down and not letting the tank get stale. I did best with frequent small water changes rather than big, infrequent ones.

What to feed them

These are eaters, and they color up and breed better when you treat them like the little predators they are. Live and frozen foods make a noticeable difference.

• Staples: frozen bloodworms, frozen brine shrimp, daphnia, cyclops
• Live foods (great if you can): baby brine shrimp, grindal worms, white worms (sparingly), mosquito larvae where legal/safe
• Dry food: some will take quality micro-pellets, but do not be shocked if they ignore flakes at first

Feed small portions 1-2 times a day. They will happily overeat, and leftover meaty food turns water sour fast. If you are conditioning a pair for breeding, bump the variety and add an extra small feeding rather than dumping in a big meal.

How they behave and who they get along with

Similis are alert, fast, and can be spicy with each other, especially males. You will see displaying, chasing, and lots of little stand-offs. A bit of sparring is normal. Constant cornering is not.

• Best setup: species-only with 1 male to 2-3 females, or a small group with heavy cover and extra females
• Tankmates: I usually do not bother. They are short-lived annuals, and mixing them into a community tends to end in stress, missed meals, or nipped fins.
• Avoid: fin-nippers, pushy feeders, and anything that will outcompete them at meal time

If you insist on tankmates, keep them calm and small, and only in a bigger tank with lots of plants. But honestly, these fish are way more rewarding when you can watch their natural behavior without distractions.

Breeding tips

Breeding is the whole reason many people keep annual killifish. They are not hard in the sense of "mystery steps" - they are hard because you are basically running a little egg-and-soil project alongside your aquarium.

They are substrate spawners. Give them a container of damp peat moss or coconut fiber (peat is the classic), and they will drive eggs down into it. I use a small plastic tub or jar laid on its side, filled with spawning medium, so it is easy to pull and replace.

• Condition adults for 1-2 weeks with lots of frozen/live food
• Provide a spawning container and leave it in for several days
• Swap containers so you always have a fresh one in the tank
• Gently squeeze excess water from the used medium (damp, not dripping) and store it in a labeled bag or container
• Incubate the eggs at room temp and check occasionally for fungus or early hatching

For hatching, you re-wet the peat/coco fiber in a small container with clean, oxygen-rich water and gentle aeration. Fry are usually ready for baby brine shrimp quickly, and that first week is where you win or lose the batch. Feed often, keep the water fresh, and do not let food rot.

Common problems to watch for

• Jumping: the number one killer in my fish room. Tight lid, no gaps.
• Bloat/constipation: happens with heavy foods like worms. Mix in daphnia and do smaller meals.
• Fin damage from male aggression: add cover, reduce males, or separate bullies.
• Fungus on eggs: usually from overly wet storage or poor-quality medium. Damp, clean medium and occasional checks help.
• Short lifespan: annuals do not live like typical aquarium fish. If you want to keep the line going, plan to collect and hatch eggs regularly.

The biggest "advanced" part with similis is consistency. Clean water, steady feeding, and a simple breeding routine beats fancy gear. Once you get your rhythm, they are incredibly rewarding fish to work with.