when they multiply to a certain number they will start to eat your plants... they breed like crazy - 1-500 in a couple of months... they hatch eggs on plants, rocks, wood - if you clean those reqularly you can slow the process down... but eventually one will get into your filter - that's when the trouble starts... they have a hatchery in there sooner or later... I solved my problem with a vietnamese loach and regular cleaning, took the filter out, cleaned the filter material in tank water really good, cleaned my sand in filter water, washed the filter wia hot water... took care of my problems... they easily get out of control...