Im aware about the surface area, i have a lot of the nitrifying rocks in my sump. I also plan on adding more rocks that are supposed to be good for nitrates Seachem De-Nitrate, im not sure if they work but heard good reviews and are worth a try.I know you where concerned about the fish seeing a predator from bellow.Fish cant see themselves looking down at the bottom they see the stand, we see the reflection because of the angle. I've heard since sand has very little surface area it really does nothing for FW bio filtration. I would think a puffer whose stomach is soft and vulnerable would appreciate a smooth glass surface more than a rocky jagged gravel. I personally feel the bad of gravel with large fish out weighs the good by far. What purpose does it serve except for allowing poop/food to get lodged in it? If looking for more biological filtration add more media, wheels, whatever and get the same effect. Like i said before, my problem with sand is it has broken several of my HOB filers on other tanks.When i ran this tank as a reef the flow would constantly blow it around clogging up the filter sock/sponge and getting into the sump where it could damage my chiller/pumps. This is not a chance im willing to take unless it is a must which it is not. I think you may have had a point with the wood/hiding spot and i fixed the problem. However im pretty dead set on the bare bottom.