How many male Guppies could live happily together in my 66 litre tank? As the tank matures I'm thinking of adding around 10 Neons so there would have to be room for them as well.
A 20 gallon tank should be able to hold all 10 neons and leave room for about 5 or 6 guppies. If you end up with mixed sexes of guppies, you need to have room for fry in the tank as well.