For most of my 10 gallon tanks I use the tetra "Whisper" submersible filters rated for 10 gallons,
I keep anywhere from 1 pr to a hundred guppies in a single tank.
being as guppies have lots of young every month it's doesn't take long to have an over abundance of them.
More fish = more water changes as something can go wrong much faster..
I keep my guppies at a temp of 78 - 82,
the warmer the water the shorter their life span, the cooler the longer they live, within reason of course.