marble starfish and puffer/ lion ?

1. hi all, couple of questions for you. first is about my marble star i dont know if its spliting or dying but it dont look good. about 2 months ago i noticed a slit starting from the outside of the body going in towards the center. the star seems to be trying to pull its self aprt at this cut because it will fold itself over and walk while holding the other part of it still. last night i moved him because of doing a water change and he is now in 3 different pieces. all of which seem to be able to move. i have just a 1 arm and 2, 2 arm parts. they respond when touched by curling the tip up. is this just a nerve or is it really still alive? should i take it out? i did a water test and my nitrite level is high .2ppm (wow) is he dead poluting the tank? but hes not mushy or falling aprt when touched. HELP!
did a 20gallon water change last night.

2. next question has to do with my recent order (last week), i got a porc puffer and a vol lion. the lion didnt eat for 3 days after geting him and woke up to find him dead and the puffer nibbling on his tail.
the puffer does seem to be interested in all the fish if they get close to it, and it starts acting like a puppy wants to bite everything. not attacking but more of playful chasing because it never swims fast to keep up if the other fish wants to get away. but regaurdless if they dont swim away after 3-4 sec the puffer will give them a nip, as if he is trying to figure out, whats this!. all the other fish seem to not care anymore and just swim away from him befor the nipping happens, but back to the lion, never saw him even try to take a pot shot at the lion, not even a nip at a fin. the lion seemed to be very defensive would turn his fins at any fish that came within 9 inchs of him. and they all seemed to understand stay far away! ive heard of the puffers nipping fins but what do you think about a puffer killng a lion outright. i am leaning towards starvation. i do observe the tank a lot, and after the lights go out till 1-2 am, i like to watch what hapens . but i cant stay up looking at it 24-7 so i cant rule out an attack. both fish where about 3.5 inchs in size. love the personality of the puffer but i started a salt tank 18months ago for the single reason of getting a vol. what do you think try again or is it doomed to end up with one of them dying within days. i watch videos of tiggers, puffers and vols all the time on utube, so i know it can be done. but do you think this puffer will let it happen. does this sound like your puffers normal behavior. thanks for the time and sorry its so long.

75gal DT, 55gal refuge/sump

 

I had a FOWLR and I had a Porc Puffer, loved to eat and when we got our lionfish the only way to get him used to eating anything was to feed him live food. the Puffer got in on it and he got a taste for live food and then anything that moved got eatten , 2 clowns, banner fish and even a lion fish that was about the same size as him, which i think let to his demise as he got sick shortly after he ate him. I would keep an eye on him because it seemed like mine turned bad quick and all they want to do is eat and since he is a nocturnal feeder and some fish arent, they will be easy pickings for the puffer at night.
no matter how much i fed him he was always hungry, always picking at other fish during the day because he wasnt fast enough to catch them but at night he had his way with them. i would monitor closely and weigh your options. good luck and hope everything works out.