the 5 legs will turn on softies over time , I learned that the hard way after losing my whole tank not to long ago
the 7 leg will become those small 3 inch serpent stars
I never knew there was any difference either till I talked with a very wise old man who has kept tanks over 60 years
I listened to him and if I see any 5 leg stars I siphon them out and just leave the 7 leg ones
haven't had issues with my softies since
I also remove the 7 leg stars if I notice I have to many ,the local pet shop I deal with gives me 50cents each for the small ones and up to $2. each for the bigger ones it's only store credit but I buy tons of stuff anyway and every little bit helps