Yes, mollies clean the tank. Mollies are opportunistic feeders, which means they’ll gladly eat anything available for them, including algae. These fish will keep your tank relatively clean and prevent algae blooms.
However, you can’t solely rely on mollies to keep your tank clean. Even though they reduce algae patches significantly, they won’t be able to eat all of them. Also, due to their big appetites, mollies produce a lot of poop, and at the same time, won’t eat fish poop.
Therefore, you’ll still need to clean the tank regularly and change the water weekly to maintain the cleanliness of the tank.