I tried dirt. Messy tryin to plant after initial set up so make sure all your plants are a go. Dirt started coming through ( I used gravel cap) and plants died after about 6 months of set up. Changed it out for flouramax, which was hell because the anoxic conditions created so much hydrogen sulfide and it really stunk. The flouramax is cheap, and after, yes, many rinses, it still was cloudy, but settles out so that's not a problem. Now my tank is running very well. Soo gravel or sand? Well gravel for me was a fail because I didn't cap to well and I replanted to much. Maybe try flouramax as a cap? Sand might not cap to well but I'm not completely sure. In my opinion, dirt tanks aren't any better than eco complete or flouramax. It's just fun to try it.