I've had a 40 gal
tank for years with a few big goldfish and some other various little fish like mollies and tetras, the only filters Ive had was a cheapo aqua-tech 20/40 on the end and a big home made sponge filter in the corner hooked to a little air pump.
The water has always been nice and clear, that's saying a lot for a goldfish tank.
I recently got a case of greenwater (dunno where it came from after all these yrs, probably hitched a ride in on some coon tail plants I tossed in there from the pet store.) so now I am trying out a UV
All things being equal with no greenwater to fight, You can probably just start out easy with a similar overflow filter like I have and a good sized sponge filter for the biological filter effect, which does a lot to keep the water clean on its own.
as Mr.Zags sez, depends on what kinda fih your keeping.
a tank full of little fish wont be as much of an issue as a pair of poopfish.. er I mean goldfish.
a powerhead in a 55 tank will make no still place for the fish to rest unless its turned down to just make the water circulate.
Some fish dont like moving water, so what kinda fish are you planning on?