I have a 5 gallon fish tank. Tiny I know. I want to get a 15 gallon when I can. My tank has been running for just over 2 months and houses 3 black skirt tetras and 2 ghost shrimp. I'm learning as I go. My question is, why is there mold growing on the inside of the lid of my tank? I have a heater which keeps the water at about 77-78 degrees F. I have never done a partial water change yet (I've been reading about how it helps with ammonia levels. I just got a test kit and the ammonia levels read between 0ppm and 0.25ppm.) I also have a filter, but it was used prior, and I suspect it may need to be changed. I feed the fish through that hole in the image. Is this the food getting stuck and gone moldy? There is also mold on the cord to the blue light I never use. I read somewhere this could be calcium deposits if you have hard water. I did use hard tap water, but I also used the proper amount of water conditioner as suggested on the bottle. Or is this mold? Is it toxic? Will it harm my fish? How do I prevent it? Do I need to clean underneath the lid regularly? Is this a sign that there is something wrong in my tank?