Yellowfin gambusia

Where they come from

Yellowfin gambusia are small livebearers from slow, weedy waters in Cuba, including creeks, ditches, and swamp edges. Think warm, hard, mineral-rich water with tons of plant cover. They often cruise right at the surface under floating leaves, picking off tiny insects.

Setting up their tank

They are tougher than they look, but they appreciate space and cover. I like a 20-gallon long for a group so the females can get away from the boys. A species-only setup is easiest.

• Tank size: 15-20 gallons for a starter group (6-10 fish). Bigger is better if you plan on fry.
• Lid: Tight-fitting. They jump, especially at lights-on or during spats.
• Filter: Sponge or a gentle HOB. They hang at the surface, so give good surface agitation without blasting them.
• Substrate: Whatever you like. I use sand or fine gravel.
• Plants: Load up on floaters (water sprite, frogbit) and fast growers (hornwort, guppy grass). Females use it to hide from pushy males, and fry use it as a nursery.
• Hardscape: Branchy wood and rock piles to break line of sight.

Water parameters they handle well: pH 7.2-8.2, medium-hard to hard water (8-20 dGH), and temps around 72-78 F. They tolerate a little warmer or cooler for short periods, but keep things stable.

What to feed them

They are micropredators with a sweet tooth for bugs. I rotate foods to keep them in great shape and to bring out that yellow in the fins.

• Live/frozen: baby brine shrimp, daphnia, mosquito larvae, cyclops, grindal/white worms (go easy on the worms).
• Dry: quality small pellets or fine flakes with a decent protein content.
• Greens: a little spirulina or veggie flake now and then keeps digestion moving.

How they behave and who they get along with

Active, surface-hugging, and a bit feisty. Males are persistent and will pester females, so keep more females than males. Think 1 male to 3-4 females.

• Best setup: species-only or with other quick, short-finned fish that like harder water.
• Possible tankmates: hardy rainbows (pseudomugils), zebra danios, faster barbs. Test carefully.
• Avoid: fancy guppies, bettas, long-finned tetras, slow fish, and delicate shrimp. They nip and they hunt tiny shrimplets.
• Inverts: snails are fine; shrimp colonies often dwindle unless heavily planted.

Breeding tips

They are livebearers and don't need much convincing. If you have both sexes and stable water, you will get fry. Gestation is roughly 4-6 weeks depending on temperature and diet.

• Sexing: Males are smaller, slimmer, with a gonopodium; females are deeper-bodied and larger.
• Fry safety: Dense floating plants are your best friend. Adults will pick off babies if they can.
• Separate drop: Move a very round, near-term female to a small, cycled nursery tank, then return her right after the drop to avoid stress.
• First foods: newly hatched brine shrimp, microworms, or fine powdered fry food from day one.
• Water for fry: keep it clean and mineral-rich. Gentle sponge filtration and small daily top-offs work well.

Common problems to watch for

• Harassment and stress: Too many males or not enough cover leads to shredded fins and hiding females.
• Soft, acidic water: You might see shimmying and poor fry survival. Add minerals and bump KH.
• Overfeeding: Fouls water and invites columnaris/fin rot. Keep feedings small and do regular water changes.
• Jumping: They rocket out through cable gaps. Cover every hole.
• Parasites: Wild-caught or pond-sourced foods can bring hitchhikers. Quarantine new fish and consider deworming if you see wasting or red worms at the vent.
• Hybridization: Do not mix with other Gambusia species if you want to keep the line pure.