to answer your third question, a sand cap is not necessary, but it looks a lot nicer. My tank is soil-based with a sand cap, but the right side of my tank keeps "exploding" with bubbles. But a lot the soil has come through the sand and is sitting on top of the sand. At the beginning it made the water REALLY messy, but now that it's water-logged enough, it doesn't cloud the water for very long when it gets disturbed. But it looks messy because there are wood chips littering the top, and it also drifted over on top of the white sand in the other part of the tank, and I'm always having to brush it off my plant leaves. I kind of wish I used only sand, or put a much thicker sand cap on it.