The way I understand it people here are recommending 2 small ones , one one each end rather then 1 large one. When you buy them they either say for up to 20 gal, for up to 30 gal ect or 150 gph or 200 gph. I just got one too and mine is 170 gph at walmart 18.86, i think. I didn't know I needed 2 until I did a search about powerheads . I have a 29 gal tank and the 170gph is plenty big enough especially once you place 2.Im no expert but I hope this helps