A livebearer tank produces fry on a rolling schedule, because females store sperm and keep delivering broods even after males are removed, so the honest answer has two halves: a plan for the youngsters you already have, and a change that stops the tank refilling forever. Doing only the first half means repeating the same scramble every few weeks, with each new brood arriving before the last one has anywhere to go.
Stop the inflow first
Separate males and females as soon as the young fish can be sexed, and keep the adults apart if you do not intend to breed. Expect a few more broods from females already carrying fry — that storage is normal livebearer biology, not a separation failure — but the numbers will fall brood by brood once no male has access. If keeping two groups is not possible, consider keeping a single-sex tank permanently, or rehoming one sex, so the population settles at fish you chose rather than fish that arrived. This single step converts an emergency into a one-off clearance job.
Raise this batch to a moveable size
Shops and new owners want juveniles that are eating well, coloured up, and large enough not to slip through nets or be eaten in a new tank, so most fry need a few weeks of growing first. A simple grow-out tank with a sponge filter, plant cover, and frequent small feeds gets them there faster than competing with adults in the display tank. Grade them loosely by size as they grow, since the biggest youngsters can intimidate the smallest at feeding time, and do not cull anything healthy just for being plain — plain juveniles are exactly what beginners and community tanks happily take.
The realistic outlets, in order
Independent aquatic shops are the steadiest outlet: many accept batches of healthy juveniles, sometimes for credit, especially clean, well-grown fish of a single variety. Call before you travel, ask what size they want, and take that as your grow-out target. Next come local aquarist clubs and societies, whose members trade and adopt common livebearers constantly, and neighbourhood groups where a bag of young guppies or platies offered free finds takers fast. Friends starting a first tank are another good home, provided you send the fish with honest care notes and one sex only, unless they genuinely want a breeding group of their own.
What never to do with surplus fry
Never release aquarium fish into ponds, drains, or waterways. Released livebearers can survive, breed, and harm wild ecosystems, and in many places it is illegal as well as irresponsible. Do not tip fry into a tank with a large fish as live food unless that was a considered feeding plan for a predator you already keep; casual disposal-by-predator in the display tank mostly stresses the community. And do not let numbers creep until the tank is visibly overcrowded, because water quality falls for every fish in it, adults included, long before the tank looks full to a casual glance.
Keeping the numbers comfortable long term
Once the backlog clears, decide what your tank is actually for. A display of one sex, a single breeding pair whose fry you have a standing outlet for, or a species-only colony with a shop expecting your juveniles are all sustainable shapes. What does not work is a mixed-sex group plus hope. A quick monthly count of juveniles, written down, tells you whether your arrangement is holding or quietly drifting back toward surplus.
The KinsLe Pet Services Directory points you to nearby aquatic businesses you can ring about surplus youngsters, turning a crowded grow-out tank into a planned handover instead of a recurring emergency.