[thincat]

First off, those tiny things sound like mini featherdusters. The other one sounds like peanut worms. Both are not a problem and more then likely came in with your lr. Your fish won't eat a chunk of garlic. Just squeeze some of the garlic juice on his food if you want to use garlic.
OCD- You will have to have someone else answer this/
The crabs are eating detritus and in turn breaking this down to another step in the nitrogen cycle. Some snails will eat this, it depends on the snail. Some only eat vegatable matter.