I see. Im not sure if you can fit a 20-30 gallon in there. The middle bar where the door meets will be a problem. If you cant then you could either drain your tank and place the sump in from the top. But if thats too much of a pain then you might just want to scrap the whole idea of a wet/dry. A extra canister or 2 would help, some canisters that would work great would be another FX5 or if you have the money, go with a eheim 2260. The 2260 will def solve your problem with bio since it can hold 17liters(i think) of media. Anyways those are your options, either go with the sump or go with another canister.