Overstocked - obviously, I hope you're wrong! lol
I may be overly optimistic, but I think of this as a mini free standing/floating deck. Have you ever seen those? They are a basic frame with cross studs. I have two cross studs of 2"x6"s underneath. I didn't use 2"x8"s because I didn't want the support studs to touch the floor - I want all the weight on the frame of the box. The box is only 8" high, minus the added bit of height for the top. The top is 3/4" ply, Hardibacker board, and tile. I certainly hope it will hold 300+ pounds, I wouldn't have built it if I was unsure. Hopefully I didn't miscalculate and make a bad mistake. I hate to say this, but I weigh over 200lbs myself, and my friend who is slightly lighter than me & I can stand on it & jump up & down. I can only hope it holds up over time, but I think it will. I'll be the first person to come here and say "you told me so!" if it fails!
If anyonme here has built their own tank stand, I'd like to know if you think it'll fail. It's also like a mini tank stand, when you think about it!
Cableguy, thanks for the tip. That's something I was wondering about. I'll ask over at the cichlid site and see if there have been problems - I don't mind coating the BG with silicone, but I'll have to do that outside - it stinks and is bad for you to breath in!
PS - in the end, I don't have to use the block at all - it's only because I don't like the lower tank to be so low that I made it. I could just put the stand on the floor...