Okay so I have a single betta in a 37 gallon cycled tank. I’ve had him for almost 2 months. I’ve noticed green algae spreading on the heater, the filter sponge, his spongebob house, and now his gravel! I’m a beginner so i’m not really sure what to do. I’ve been scrubbing the algae off with an aquarium brush but otherwise I don’t know what to do. Is it harmful to him? Is this natural?