Glad you asked. If you read the fine print, the Fluvals have two ratings on them. One is the pump capacity, proudly stated in prominent mumbers. The other is the "filter circulation" wich is significantly less and also on the box. Never saw an employee, website, or buyer ever take notice of this, myself included. So here are the numbers right from the fluval owners manual:
Fluval 104 pump = 125gph, filter circulation = 85 gph
Fluval 204 pump = 180gph, filter circulation = 110 gph
Fluval 304 pump = 260gph, filter circulation = 185 gph
Fluval 404 pump = 340gph, filter circulation = 225 gph
The owners manual does not explain why the filter circulation is only about two thirds of the pump output. Is there bypass, where flow goes around the filter so that the biologic baskets have more time with the water? Or does the pump flow out of the nozzle go down when the filter baskets are filled? I don't know.I measured my output, but I added hose length to my fluval, so my measured actual output is lower than someone with shorter hoses and is less than what fluval states. So wich number is the correct one to use???
Don't get me wrong, I love my Fluvals. Have three of them. I just haven't figured out what the ratings mean.
You are both concerned with adequate filtration, and with too much current. I would go with the larger filters, I don't think the fish will mind the current. If its too much, put the nozzle on one end, and point it along the back wall at the other end. there will be a strong current along the back at the top, but gentle current everywhere else.
And like everyone has stated before, you will love having a fluval canister.