I don't keep Discus but I do have 7 Peruvian Altums in a 75g that produce a lot of waste. The sand quickly gets accumulated mulm on it. I clean this in one of two ways:
1 - I use my regular gravel cleaning equipment and hover it over the substrate. The mulm gets sucked up pretty easily during water changes.
2 - Between water changes, when the tank looks bad, I use a HOT magnum with a micron filter and stick the end of a short piece of tubing onto it. I connect a gravel vac to the tubing and let it hover above the substrate. This method works very well and doesn't require adding any water.