Guppies are livebearers with no off switch while males and females share water. A single female can produce a batch of fry roughly every month, each batch can number in the dozens, and those fry are breeding age within a few months. Left alone, the maths does exactly what your tank is showing you. The good news is that the control method is simple — it is just not instant.
Separate the Sexes, Properly
The only reliable stop is physical separation: move every male into a separate tank, or rehome the males through your local aquatic shop or fishkeeping group, which commonly take healthy males. Sexing is straightforward in adults — males are smaller, slimmer, and far more colourful, with a pointed anal fin modified into a gonopodium; females are larger, rounder, and plainer, with a fan-shaped anal fin and often a dark gravid spot near it. Check carefully under good light, because one missed young male restarts the whole cycle, and young males colour up late — recheck the female tank weekly for a few weeks and move any late developers out as they show themselves.
Here is the part that catches owners out: separation does not stop births straight away. Females store sperm and can fertilise several successive batches from a single mating, so fry will keep arriving for months after the last male leaves. That is normal, not a failure of your separation. Plan for it rather than being surprised by it.
Handle the Fry You Already Have
Decide your fry policy honestly and apply it consistently. Many keepers let the adults and dense planting do natural population control by not rescuing fry — in a well-planted community tank, most fry are eaten, and removing breeding traps, breeder boxes, and fry refuges lowers survival sharply. If you do raise fry, separate the young males as soon as they can be sexed, at a few weeks old, before they can breed with their sisters and mother. Grow-out space fills fast, so line up your outlet early: aquatic shops often take young guppies in batches, and local fishkeeping groups rehome them readily. Never release tank fish into ponds or waterways — it harms wild populations and is illegal in many places.
Cut feeding back to what the fish finish in a couple of minutes. Overfeeding does not just cloud the water; well-fed females produce larger batches, and excess food fuels the waste load of a population that is already outgrowing the filter.
What Not to Do
Do not add a predator to solve it. A larger fish bought to eat fry usually stresses the adults, eats unpredictably, and becomes a stocking problem of its own once the fry run out. Do not keep buying bigger tanks to chase the population — separation in the tank you have beats a larger tank of mixed sexes every time. And do not rely on temperature or water tricks you may read about; none reliably stops livebearer breeding, and unstable water harms the fish you are keeping.
As the stored-sperm batches taper off over the coming months, your female tank settles to a stable, non-breeding group. Keep a simple weekly tally — fry spotted, young males moved, fish rehomed — and share those checks in KinsLe's Care Teams, where each household member's task is timestamped, so the slow wind-down is monitored by everyone rather than forgotten until the next surprise batch.