Hi, Captfish! I'm planning on the sump being 55 gallons to accommodate a 125 gallon "main" octopus tack, as well as a secondary (90 gallon) yet-to-be-decided octopus or cuttlefish tank. I'm planning on a larger than usual sump because I am going to go ahead & attempt a faux tide-pool, & I want to ensure that I've more than enough water to account for evaporation (& the extra oxygenation). If you can picture the set-up, the tide-pool is going to extend 48" off the rear rim of the 125, with 3" surrounding either side & the front of the rim with which to secure a (possibly) Plexiglas 20" wall, which itself will support a secure lid with lighting. Considering the over-lap of the tide-pool, I think a 90 gallon beneath it would work well; it'd serve as that much more of a buffer with regards to water stability,& aesthetically speaking would fill a rather large void, especially as its to be placed as a centerpiece in a large room..I'm well aware of quarantine & isolation practices, but from what I've read, & PLEASE correct me if I'm wrong, but I've been under the impression that disease (bacteria in particular) doesn't spread readily amongst cephs. That fact, along with reading here of other shared sump successes, has lead me to this current blueprint. Please, I welcome any input...