I was going to go 75, but to go 90 it is only 4 inches deeper, and I am really glad I decided to go 90. I was worried that 24 inches would be to deep to work in, but it isn't that bad at all. If you don't already have the 75 you might look at the 90 predrilled too. Also, I went the 50 gallon trash can method for making water, the one I got which was a nice heavy duty one, didn't hold up well when it was about 3/4 full, so don't fill it up too full. I had the liverock out of the water for about 8 hours in rubbermades and seem to have avoided any noticable die off. Have fun.