You’ll need a 10-gallon aquarium, minimum. Guppies should be kept in a school of six or more, and you should shoot for one male for every two females. That means you’ll be choosing your number of fish, and your tank size, by the interval of three. In other words, you can have six, nine, twelve or fifteen guppies, but not seven or ten or eleven. Keep in mind this is a suggested best practice. Things go off the rails quickly with these little guys, so don’t stress yourself out if your ratio is a little off.
Guppies are also one of the few fish where the “one-inch-per-gallon rule” applies. I generally tell people to disregard this oft-repeated rule of thumb, as it doesn’t make much sense for most fish species. However, guppies are little fish with a low bio-load, and they are around an inch long. So, we can use this “rule” to decide on tank size.
Putting all of this together, say you decide you want 15 guppies. You will need at least a 15-gallon tank. If you want 30 guppies (!) you will need a 30-gallon tank. If you only want a small school of six, again I’d go with a minimum of 10 gallons.
Guppies are also live-bearers and prolific breeders, so you may want to over-estimate on tank size a little, as it may not be long until you see little guppies puttering around. This is the reason you shoot for that 2:1 female-to-male ration. Male guppies tend to get a little annoying, so having more females means the stress is spread around a little more.