My brother in law use to have 20 fish tanks ranging from 5 gallons to 125 gallons all full of fish, fresh and salt water. We had a massive storm a few years ago and the power went out for two weeks and his fish died and he lost hope and interest after that so her quit doing it. He gave me some gravle that sat in a fish tank for close to five years with out draining the water or even cleaning it so it all dried up. The rocks are filthy and needs cleaning. I've boiled them three times and can't get them clean, what would be the best way to clean the rocks? They have a film over them and some have algae on them. Any ideas?