You need more sand. I have like 1.5 to 2 inch cap on mine. 30 gallon. Didn't have much problems. I have one 2.5 gallon that I didn't put enough sand in, and it got all brown for about a week. I had to dump it and cap with about an inch.
Also, soil takes a lot of patience. Once plants get going they will go, but don't expect an immediate response.