What material can be used to clean inside a fish tank?

A piece of plastic netting used for bagging potatoes or oranges works well on algae stuck to the glass. Be sure to rinse it well in clean water (no soap) or salt water solution first. Hot salt water is safe for cleaning the tank and gravel itself, but rinse well if you have fishes intolerant of salt. There is no detergent safe for cleaning aquariums. There is always the risk of residue that could kill your fish. Alternatively, rinsing gravel well with running tap water, mixing it periodically by hand until the water flows clear, will also work. A product like Stress Coat will condition water, remove chlorine, and eliminate any risk of chlorine from the washing affecting some more sensitive fishes. If you are tearing down the whole tank to clean it, the water you prepare for the fishes will be enough. There is no need to put de-chlorinator in the gravel, in any case.