I have a DIY sump, tons of media, a 2200GHP(Quiet One) pump, other pumps and powerheads with filters, along with subtrate- 2 barrels of natural sand and gravel.
galloons is hard to figure out, but a conservative guess would be at least 750 but more realistically 800-900gals.
I could but I would never feel safe with the tank after that and I would hate to wake up to 900 gallon a water on my floor and as far as the wall thing as suggested above I don't have the time or money to be knocking down wall to make room for a fish tank right now I wish I did but I don't