There are a couple choices you can put right in the biocube and not have to hang it off the back. Sapphire aquatics makes 1 or 2 (I'm not sure what is currently available, but there used to be a single pump one and a recirculating model), CPR makes one, The Tunze 9002 will fit. FWIW, I tried a couple of those on my biocube and none of them work all that great. If I was still running that tank, I'd think of doing GFO. Sapphire makes a cheap reactor that will fit as does at least one other manufacturer I can't think of right now. The last 9 months or so I ran that tank, I just ran it skimmerless. I didn't do water changes all that often either but when I did, I did big ones.