My tank is stuffed to the top with rocks.
I have a rock wall almost all the way to the top in the back of the tank. My rocks touch the sides too. Almost all the way up. I actually have very little room in my tank for any swimming. But, the fish I have do very well. There's tunnels and holes and caves all over in my rockwork. My 6-line wrasse loves that. They like twisting and turning in and out of the rocks. My clown fish is a total dork and only swims back and forth on the front glass. So, he could care less about how much rock is stacked where. And my goby stays on the bottom in one of 3 or 4 different caves I built specifically for him. I've even got rocks touching the front glass.
Rocks! Rocks! Rocks EVERYWHERE!!! :mrgreen:
I think it depends on what type of fish you plan to have in the tank. If you have larger species, then they are going to need 3 or 4 feet of swimming space to keep in shape. But, if you have rock and cave dwellers, as I do, then I'd say-- FILL ER UP!!