Sometimes I use towels (regular sized bath towels that I only use for fish) to isolate areas of the tank or block off parts. Its not perfect and they are tough keep in place underwater especially if there is strong current, but I think it would help just to even eliminate some of their escape routes away from the net. For my big fish I usually hand catch them or use the towel to scoop them up, but that's probably not the best idea for moving discus especially with rays in the tank. Good Luck