Another thing. U could use
LR rubble instead of the rings. And I don't know how old ur fluval 305 is, but even the old fluval 405 I have on my
FW tank has a flow adjustment right where the hoses connect to the canister, just above the disconnect. My new 305 has it as well. Down the road u might consider using chemipure Elite instead of carbon. Thats the good part of using the canisters. There r many options