Topsoil and potting soil are not the best items to use to cover a background. They are both far from "clean". They are both filled with insect larvae and eggs that could potentially muck things up. They could also potentially cause a molding issue within the tank. If you want a darker color, you can try a milled sphagnum peat moss. The downside to that is normally the gnat eggs that are ever present in it, so you need to be careful.

You could possibly boil either of those to kill off any unwanted pests, then allow it to completely dry out, but I am not sure you want to be boiling soil indoors. At least not without a whole bunch of air fresheners around lol


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