Since our basement had a finished floor, we tore out the spot in the fish room and added pond lining around and up the wall to hold over 300g of water. However, we also have a floor drain, so it will never hold that much. I love my sink although I wish it were bigger! All around, I would love a fish room twice as big. I am always adding tanks and it is the only place hubby will not find them. :)
We used green board for the framing (like in bathrooms). Wish I had an outside vent though. That would be perfect! Humidity can be an issue when the chiller starts up in the summer.
Does a minifridge full of fish food count? It is actually part of my auto-feed system, but really helps keep my phyto and brine eggs and such handy since that is where I culture. Also holds food for my fish tanks in that room. Less up and down for feeding.
Don't forget to pipe in that RO/DI unit too. Very handy.