I wouldn't say that Dwarf frogs are particularly filthy so if you were to get a small submersible filter it would probably work ok. If you were to have an undergravel filter it could also work. I once had a pair of frogs in a one gallon tank with under gravel filtration and they really thrived. I think it depends on the frog. Some will be more active and therefore particular about their water current but some are going to hide no matter what you do and only come out at night making it hard to tell what they prefer. Also the tank size depends on what size frogs you bought. Usually at local pet stores they have Medium, Large and Extra Large dwarf frogs for sale. If you have 4 of the medium variety you could have a smaller tank than 5 gallons and do alright. If you have four of the extra large variety you might even want a 10 gallon. It depends on how large you want them to get and how heavily planted you intend to make it. Like fish, the frogs won't attempt to outgrow their environment but it will put stress on them if they were supposed to be bigger and you give them a small space. They need hiding places to stay unstressed. I reccomend plants vs. decorations like caves becuase you still want to be able to keep track of them. It's easier to look around your tank and locate everyone in plants than try to figure out if they are hiding inside a castle or cavern where it would be impossible to see them. Plants ranging from 3-9 in would be good if you are having a 5 gallon.