Sailfin molly

The sailfin molly reaches about 12.5 cm standard length, males carrying the tall dorsal fin the species is named for. It is native to southern North America and Mexico, from the Carolinas to Veracruz, where it lives in heavily vegetated coastal streams, ponds, swamps, canals and ditches, and it is particularly associated with brackish water carrying heavy growths of algae and phytoplankton. Introduced populations are now established in countries as far apart as Kenya and New Zealand. Wild fish are rare in the hobby; almost all stock is mass-produced in the Far East and Eastern Europe.

Hard, alkaline water is not optional for this fish. Keep it at pH 7.0 to 8.5 and 15 to 35 dGH, at 21 to 26 °C, in a tank of at least 76 by 38 cm, roughly 87 litres. In soft or acidic water it weakens quickly, and the warning signs are shimmying, fungus and clamped fins. Salt is often recommended and is not actually necessary; it is the hard minerals, calcium and magnesium, that keep this species healthy long term.

The diet should lean vegetarian. Wild fish eat almost nothing but algae and plant material, and while farmed stock accepts most foods, blanched spinach, vegetable flake and algae need to be part of the routine, since too little greenstuff impairs dorsal fin development in males. Soft-leaved plants may be eaten, and a little green algae is worth leaving in place as grazing. Males spar with each other and may pick on smaller fish as they mature, so keep one male to two females. Breeding needs no encouragement: gestation runs about two months and up to 50 relatively large fry are produced, able to take brine shrimp nauplii from birth. Wild-type fish are noticeably less likely to eat their own young than the domestic colour forms.