The 304 would not be the only filter on the tank 80gJoe. I said in the original post that I already have a 350 mag and an emp 280 on the tank. With those three filters I would have 890gph, which is enough IMO.
I have a 304 on my 55gal tank (along with a couple HOBs and a powerhead) and it works pretty good.... it will give you a very large amount of biological filtration. I have run into a few problems... I've had a locking clip break on me..and the hoses could have been designed in a better way. If I had the money at the time, I think an eheim would have been a better choice, but the fluval works pretty good. I can get the 304's decently cheap, so I might buy a couple more in the future.
