Plains topminnow

Where they come from

Plains topminnows are a native North American killifish from the Great Plains region (Nebraska, Kansas, parts of Colorado and nearby). In the wild I think of them as "ditch and backwater" fish - slow creeks, ponds, oxbows, and weedy margins where the water can get warm in summer and pretty chilly in winter.

That background explains a lot: they like cover near the surface, they are tough once settled, and they do best in tanks that feel a bit "natural" instead of sterile.

Setting up their tank

Give them floor space and surface cover more than height. A 20-long is a sweet spot for a small group, but a 10-gallon can work for a trio if you stay on top of maintenance. They are jumpers, so a lid is not optional.

• Tank size: 20-long for 6-10 fish is comfortable; bigger is even easier
• Filtration: gentle flow (sponge filter or a baffled HOB). They do not love being blasted around
• Hardscape/plants: dense plants up top (guppy grass, hornwort, water sprite, frogbit) plus some open swimming lanes
• Substrate: anything works, but dark sand/gravel makes their colors look nicer and calms them down
• Lid: tight fitting, cover gaps around airline/heater cords

Water-wise, they are pretty flexible. Neutral to moderately hard water is totally fine. I aim for stable, clean water over chasing exact numbers. Temperature in the low-to-mid 70s F works well, and they can handle cooler water too. If you keep them warm year-round, they will still do fine, but you may see a shorter lifespan and more nonstop spawning behavior.

Feeding

Plains topminnows are surface pickers. They will eat flakes and small pellets, but they really come alive on small live and frozen foods. If you have ever kept other Fundulus, you will recognize that "always hungry, always hunting" vibe.

• Staples: good quality flake, small floating pellets, crushed pellets for smaller fish
• Frozen: daphnia, baby brine shrimp, cyclops, chopped bloodworms (as a treat)
• Live (great if you can): mosquito larvae (where legal), daphnia, grindal worms, baby brine shrimp

Feed small amounts once or twice a day. If you dump in a lot at once, the bold fish will hog it and the shy ones hang back. I like to spread food across the surface so everybody gets a shot.

Behavior and tankmates

They are active, curious fish that spend a lot of time near the top. Males can be pushy with each other, especially in small tanks or in bare setups. In a planted tank with a group, the aggression usually turns into short chases instead of real damage.

• Best group size: 6+ so the attention is spread out
• Sex ratio: more females than males reduces stress (something like 1 male to 2-3 females)
• They jump: sudden spooks, feeding time, even "because" - keep that lid on

Tankmates should be peaceful and not nippy. Avoid fish that harass fins, and avoid big fish that view a 2-inch topminnow as a snack. If you want a native-themed tank, they pair well with small shiners and peaceful darters that stay lower, as long as everyone likes similar temps and you have enough space.

Breeding tips

They are egg scatterers and will spawn readily if they are well-fed. The catch is they are not great parents. Eggs and fry disappear fast in the main tank unless you have a ridiculous amount of plants.

• Give them a spawning mop (yarn mop) or a thick clump of fine plants near the surface
• Condition adults with frozen/live foods for a week or two
• Check the mop/plants daily and move eggs to a small container or a separate rearing tank
• Use gentle aeration and clean water for the eggs; a drop of methylene blue can help if fungus becomes a thing in your setup
• First foods: infusoria/microworms, then baby brine shrimp as soon as they can take it

If you want "easy mode," keep one male with a couple females in a breeder setup and pull the mop every day. You will get a steady trickle of eggs instead of one big spawn, which is nice for not getting overwhelmed with fry.

Common problems to watch for

• Jumping: the #1 killer. Tight lid, block gaps, keep water level a little lower if you have a rimless tank
• Shredded fins from male sparring: usually fixed with more plants, a larger group, or a better male-to-female ratio
• Getting outcompeted at feeding time by faster midwater fish: feed at the surface and spread it out
• Skinny fish: they look "fine" but lose weight if the diet is too dry-only. Add frozen/live foods
• Ich/velvet after shipping or temperature swings: quarantine new fish and avoid rapid temp changes
• Egg fungus in breeding setups: improve cleanliness, add gentle aeration, and remove bad eggs promptly

If something seems off, look at the simple stuff first: are they spooking (no cover), are they getting pushed around (too many males), and are you feeding foods that match a surface hunter? Fixing those three clears up most headaches with this species.