when i first started fish keeping i kept feeder goldfish.I feed then worms because it was funny to see 2 goldfish play tug of war on it and some got away.1 month later i cleaned the whole tank out for tropical fish.i pulled out 2 earthworms and were still alive.The air must get stuck down there if you have a lot of substrate.I noticed theres some air in my sand in my new saltwater tank.I also heard there good to clean youre tank out also.