Is that a type of leechNot actually detritus worms or any kind of flatworm, those are suction feeding worms. They are harmless and should be eaten by most things that see them. They feed off of microalgae, detritus and biofilm. To lower their numbers you can clean the tank/filter to remove their food and feeding less can help too if overfeeding has taken place.
Nope, it's a type of freshwater bristleworm that make up the family Aeolosomatidae.Is that a type of leech