I think it is pretty normal. You might want to soak the soil before you put it in th tank. Change the water a few times. You might want to use a less porous cap like sand.
I like sand too. I am working on which is better sand or gravel. I like the capping properties of sand, I like the look of sand. But from what I know about gravel, it is allows the nutrients from unconsumed organics to fall back through into the soil, thus completing the cycle, and allowing the soil to remain rich for many years. I believe I can achieve the same result with sand by using snails and worms, but I have not completed my studies yet. I will open a new post on this subject.