[Dutch]

unless there taking your tank over just leave them they are part of the natural reef anyway and they help ( in moderation of course ) to control nutrient levels, feeding on waste food etc. I have a few in my tank and they only come out during feeding time eating the extra food. If they are taking over your tank try using the flat worm trap starve tank for a few days then put in trap with a little food inside this takes some time to reduce numbers but does help.