Welcome to the forum Fern Fish. The cause of cloudiness can be an algae bloom, which looks like it has a slight green tinge, or a simple bacterial bloom that is likely to be a whitish color. On a mature tank, the algae bloom is more likely and would be due to excess light. I keep trying, without any success, to cultivate what is known as green water algae. I can never get enough light but the light needed is like some sunlight or very intense and long time period light.