Why the ratio exists at all
Male guppies, platies, mollies and swordtails share one defining habit: courtship is not an event, it is a full-time occupation. A male in breeding condition approaches females constantly, displaying, chasing and attempting to mate through every lit hour. One female cannot rest from that; she hides, stops feeding well, and may abort fry or succumb to the sheer wear of it. Two or three females per male splits the attention so each female gets long unbothered stretches, and the whole tank's behavior settles into something that looks like community life instead of pursuit.
The working numbers for each common species
Guppies do well at one male to two or three females. Platies are a touch more relaxed but still best at two females per male. Mollies, bigger and more persistent, reward the three-to-one end of the range. Swordtails add a second problem — males fight each other — so a swordtail tank usually runs best with a single male and several females, or a group large enough, in a tank long enough, for rival males to hold separate ends. These are starting points rather than laws: an unusually relentless male guppy may need the female count raised around him specifically.
Sexing the fish you actually have
Ratios fail when the sexes are guessed wrong. In most livebearers the male carries a gonopodium — a pointed, rod-like anal fin — while the female's anal fin is a rounded fan. Male guppies and swordtails announce themselves with color and, in swordtails, the tail extension; female mollies and platies can look deceptively bright in shop tanks. Juveniles all look female until males mature and the anal fin transforms, which is why a tank bought as mostly female can turn male-heavy over a few months. Recheck the juveniles monthly as they grow and move surplus young males to another tank or back to the shop before they join the pursuit.
Fixing a tank that is already out of balance
The fastest correction is adding females, but only if the tank has the room and filtration for them. Where it does not, remove males instead — rehoming two pushy males restores peace faster than crowding in more fish. Dense planting, floating cover and broken sight-lines buy the females rest in the meantime, and a dark background corner where a tired female can sit out of the courting lanes makes a visible difference within a day. Watch behavior after the change: females feeding in the open at midday are the clearest confirmation the ratio now works.
Ratios and the fry you did not plan
A balanced livebearer tank is also a productive one, and the ratio conversation is incomplete without the fry plan. Some keepers let the tank self-regulate, with most fry eaten and the strongest few surviving in the plants; others net out pregnant females or raise fry in a grow-out box and move the young males on as soon as they can be sexed. Either way, every surviving generation re-rolls the ratio. Booking a monthly recount and juvenile sort as a recurring duty in KinsLe's Care Teams keeps the balance from drifting silently, so the tank you corrected stays corrected as its own babies grow up.