How do I prevent fungal growth on my pebbles and cave in my betta tank? I just cleaned out the tank because there was black fungus growing on the pebbles and green fungus growing on the cave. I only have one betta fish living in it. The tank is 1 gallon and has a filter and light. Also, whenever I change it's water, I keep the tank in the garage for 1 hour so the water isn't too cold for my betta.