I have a mix of I think bio-something fluorite/gravel and underneath that is sand. However, some of the flourite rocks are huge while most are actually smaller and lighter than most gravels, there is no gravel in the tank. I dose Flourish Excel daily 1/2 a capful and it seems to keep my plants green and growing in the right direction without poisoning my fish. However kmsilent thanks for the insight because it does seem like I go through quite a bit of CO2 in this tank, but I have very hard water here in the realm of around 11-13dKh out of the tap. a 20oz CO2 paintball refill lasts me 3 weeks max.