In answer to your original question, if you have an unwanted hitch hiker in your tank, you must approach it as if you are one of the moderators on this forum. first identify the specimen, then assess the situation (how many are there, is it an infestation, how dangerous this specimen is to the ecosystem), you then decide to either rid your system of said hitch hiker, or allow it to be part of your system. If you chose to get rid of it, the best thing to do is banish it to the sump or the toilet, never to return.