The issue isn't that you're not providing food. In a 55 gallon tank, there's a ton of surface area for biofilm to grow. Tons of room for bacteria to develop in the substrate. That's an endless food supply.
Once you have them they're hard to get rid of. People say oh you're just feeding too much. Even in a a heavily planted 55 gallon that had no fish and therefore no food added they maintained a population of about 17 billion.