Can anyone help with this? I've got a well established tank but a black algae seems to be taking over the lava rock. There's small spots of it here and there but the rock and surrounding gravel seems to be affected the worst. Any hints on what it is and how to get rid of it greatly appreciated. We've used an algae treatment but it's had no effect. Any fish suitable for a community tank that might help? We've got ottos already but they aren't doing anything! Fish wise we've got platys, guppies, tetras, assassin snails, shrimp and Cory's. Thank you ?