No, they aren't bad as long as they are in moderation.. I mean like if the sides of your tank is covered with them then you have a water quality problem and are probably overfeeding by quite a bit.. In small numbers they are fine, will take care of some extra wastes( and produce some as well ) and can be a little snack for some fishies in the tank or any snails. I actually like the idea of them there because they will keep my guppies hunting for food all day.