I left my aquarium in the care of a friend while I was away for 2 months and when I returned I had a horrible outbreak of blackbeard algae and green hair algae. The blackbeard algae was on everything and had killed all but one of my plants. I also had green hair algae growing on everything and flowing in the water so thick that it was trapping and killing some of my little fish. I had to break everything down, put a new piece of driftwood in, put fake plants back in until I can assure myself that it isn't coming back, but I do have one amazon sword left that I was able to salvage. The only blackbeard algae that remains is a small amount on a piece of gravel rock here and there. What is the best way to get rid of the algae on the gravel rocks so it doesn't grow out of control again?