You could put mineralized topsoil under the sand, and have a nutrient rich substrate as a result. It really depends on how much money you are willing to spend on substrate. But, the mineralized topsoil substitutes your work for giving money to a store.
Honestly, i just bought a condo and will be pretty much emptying my checking account to make the down payment. So i don't really want to spend all that much on dirt. About how much would a layer of the top soil cost for a 75 gallon?
thanks for the help