05-04-2010, 10:07 AM
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The issue with sand is compaction. I've not heard of fish actually eating it; some bottom fish sift through it for food, but the sand comes out through the gills or is spit out.
Water must be able to flow through the substrate; bacteria live in the substrate and they need oxygen plus the water transports nutrients to the plant roots. When the substrate compacts water is impeded and sometimes stops flowing through. This kills the aerobic bacteria (and stops the transport of nutrients) which causes anaerobic bacteria to increase creating other issues with nitrogen gas, and the plant roots rot, further adding to the problems. Any substrate can compact if not properly cared for, but as sand is so fine a grain it is much easier for this to occur than with regular gravel. The deeper the substrate the more likely it is to compact. In a small tank as you mention, having a thin (relatively) layer of sand, say 1.5 inches, and raking or poking it regularly, would work. Gravel is frankly easier to handle. But that is up to you.
Plants need food, which are nutrients, plus light ibn order to grow. It is possible to have a balanced system where the fish, fish food, and minerals in the water (replaced via water changes) provide what the plants require in balance with the light. But most of us find that this is insufficient, so we use a liquid fertilizer once or sometimes twice a week. I recommend Seachem's Flourish Comprehensive Supplement for the Planted Aquarium. It contains most all of the nutrients required (the others come from the fish and biological processes) and in the relative proportions. In a 2.4g tank it would only take a drop or two per week.