With which filter ?

spong filter

Depending upon how much air you´re going to pump through that sponge filter and how large the sponge filter is you can have anywhere from 8-10 triads up to 100 fish in a 5 gal aquarium; the larger the pump and the larger the filter the higher biomass it can sustain. Avoid smooth surface sponge filters, if possible purchase grooved ones, they offer more surface.

Rule of thumb 1 inch of fish for one gallon although this changes with filter/air so on, but less is best.... also guppies love to nibble on plants... algae on them as well... so bare tank not best for any fish really.. in my 10 gallon I have 5 fancy guppies, 3 left over feeder guppies (for rescued turtles which started the WHOLE thing, lol) 5 neons, 3 catfish, 3 danos and three jap snails, and I feel it is a bit overcrowded...(one underwater filter and one hanging on outside, air stone) and plants... best Sherry