We have had a figure 8 puffer for about 3 months now. We have a 55 gal, brackish tank set up, crushed coral substrate, salinity is ~1.003-4 right now, pH 8.0, nitrates and ammonia both 0, and we do ~25-30% water changes weekly. This is a tank that my fiance has had set up for about 4 years. The other fish in the tank (which I know both are not true 'brackish' fish, but have been in these conditions for their whole life, and are growing/thriving: one 4" clown loach (had this guy for about a year and is thriving and growing), one ~7" banded leporinus (had for 4 years) and one ~8" red tail tinfoil barb (~4 1/2 years) and one baby tiger barb who is now 'schooling' with the clown loach (only had for 2 months as well).
We have been varying the puffer's diet, giving him lots of seafood like clams, shell-on shrimp, frozen krill, frozen crab legs. I also always dose his food with Zoe Marine vitamins. He has quickly become my favorite fish in the tank!
My fiance (about a week ago) purchased a columbian shark, which he did without doing research

just knowing that he was a cute fish and brackish. So, when he got home and I did the research, we realized how big this fish will get, and he returned the shark (under my demands!) Saturday. My question is: when we first got the puffer, he ate voraciously, everytime I put food in the tank. It seems over the past week or so, he's 'gone off' his food... Because of the bigger fish, I usually hand feed him with forceps, or put his food into a large net and he swims right in and feeds by himself in the net. Could it be that the addition of the shark caused him to go off his food?? He's accepted none of the shrimp/ crab/ clam that I've offered him the past week, he just swims over like he is interested, but doesn't actually eat. He only pecked and eaten a little bit of some freeze dried krill I put in there as a last resort yesterday and tonight, all he would take after being offered crab was a couple of freezedried bloodworms... his belly is still bright, bright white. The only thing I've noticed is that he always did some flashing off the heater (he seems to love to rub his little body on it) but have seen no signs of any external parasites or ich... so he's still doing that flashing type behavior, but I always thought that that was normal in small doses for puffers. (Also, his beak looks good and doesn't appear overgrown)
Any help? Could he just be a little emotional over us putting the shark in the tank and then getting rid of him??

He used to zip right to the front of the tank whenever I went to feed the other fish, and I haven't seen this behavior in quite some time. It especially worried me tonight when he seemed to only 'push' the freeze dried krill... I haven't seen him take a good healthy bite of anything in quite some time. Any suggestions?