my guess is just change about 1/4 of the water, if u have a nice HOB filter on it you really dont need to worry about it because my little five gallon never really got dirty and it had a UGF on it. and it had some goldfish in it.
If you get a Dwarf Puffer you won't have to worry about that growing guppy population either. DPs will tear little guppy fry to pieces. Bettas will also do the same.
Simething wrong with the tank it smells.. a lot and it cant be clraned with just water So we are going with reptile/amphibian or a spider. Cant afford a filter LOL and no plug left : ( so it'll be a spider or a frog