Room for a real colony
The comfortable range for a 40-gallon sits in the mid-twenties to mid-thirties of adult fish, and the extra length and surface area change the character of the keeping: males can display and drift apart instead of crowding one corner, females get genuine refuges from attention, and the water chemistry moves slowly enough that weekly care keeps it steady. Even so, a breeding colony expands to fill whatever room it is given, so the number is a managed population, not a purchase. Start with around fifteen healthy adults and let the tank grow into its stocking over the first few months.
Founding the colony
Buy or accept the founders in a ratio near one male to two females, choosing males for the colours and tail shapes you want to see repeated — because in a colony, the males you start with shape the look of everything that follows. Quarantine new arrivals where you can, and add them in small batches rather than all at once so the filter adjusts gradually. A 40-gallon also supports keeping two loose bloodlines, one group added now and unrelated males added later, which keeps the colony vigorous instead of letting it inbreed its way downhill over generations.
Planting as the birth-rate dial
In a tank this size, planting is how you set the colony's growth rate. Thickets of stem plants, floating cover and moss mats at one end of the tank create a nursery where many fry survive; open substrate and a bare mid-water column elsewhere let the adults hunt the rest. Want slow growth? Plant the nursery smaller. This dial matters more than any stocking number you read, because it operates every single month without you lifting a net — and it is far easier to adjust than a tank that is already overrun.
Feeding and filtration for dozens of small mouths
Dozens of guppies eat like one much larger fish split into pieces: small meals two or three times a day, finely crumbled flake or micro-pellets spread along the surface so the whole colony feeds at once, with powdered food drifted into the planted end for the fry. Filtration that would suit a lightly stocked 40-gallon will strain under a full colony — oversize it modestly, protect the intake with a sponge so fry are not drawn in, and keep weekly partial water changes non-negotiable. Keep the flow gentle, too: long-finned males tire quickly in a strong current.
When the tank looks full
The colony has outgrown the tank when feeding time turns to chaos, nitrate climbs between changes, and young males colour up poorly in the crowd. Your outlets are other keepers, local shops that accept juveniles, or a second tank splitting males from females — decide which outlet you will use before you actually need it. For the thinning rhythm itself, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library — well before the filter falls behind.