Weird Trimac issue - lethargic

A little background:

-Male trimac bought from Rapps in March 2013 @ 1in. Currently approx 10in
-kept solo in a 125gal. Temp @ 79-80. Two AC 110s for filtration. I change 75gallons x2 weekly. Ammonia & nitrite always zero, nitrates around 5.

Starting 12/26, my trimcac hasn't been himself. He has been indifferent to food - he will still eat, just not begging & eating like a pig like usual & for the most part he stays motionless in one spot or resting on the bottom. He used to always be out patrolling & flaring at me when I enter his room. Nothing has changed in his diet - FD krill, FD blackwom cubes & pellets. Water chemistry has been normal - I've done several tests. The only thing in his tank that has changed is I have had a larbe bllom of malaysian trumpet snails - they've always been present, but they just exploded.

I have seen no symptoms other than his overall lethargy, His color was off for 2-3 days like he was stressed, but I couldn't find the cause. Water chemistry fine, no stray currents, no chemicals sprayed in the house. I don't like the idea of shotgun medicating, but I did run a course of General Cure to see if it was anything internal & so far no change.

I was just getting ready to move him to a 90gal to keep as a wet pet. I want to use the 125 to grow out some red tiger motas in hopes for a pair.

2 Questions:

1. Is there anything I might have missed w/ the Trimac? I'm pretty frustrated as I can't seem to find anything that would have caused this overall lethargy. He literally went from a glass banging food hog, to a total lazyass overnight.

2. What would you do with the 125 in regards to sterilizing it? I want to get the RTMs soon, but I want to make sure the tank is safe. Would you nuke the whole tank w/ clorox & start over? Looking for advice here.

Thanks for the response Tom. No bloat & no thinning. The MTS outbreak started a week or two before his attitude changed. MTS have always been present in his tank, just not this amount.

I heard a fish hit the lid of the tank the night before he started acting strange - I figured it was my vieja tank as they can get rowdy, but now I'm thinking he might have gotten spooked & crashed into the lid of his tank. I wrote it off the next day b/c I didn't see any scuffs or damage to him. Could this be a prolonged effect of that? Like concussion syndrome. Grasping at straws here as I'm pretty much at a loss.

What do you think I should do w/ the 125 after I move the trimac? What kind of sterilization would you do if any?

Sounds good. I'll give the meth blue a shot. My first thought when I saw the attitude change and all my water tests came out clear was hex, but I've never seen that come on as quickly as this. Also, with hex the fish seem to stare off into space and my fish is alert...really has me stumped.

No never saw any signs of hex - white poop is a dead giveaway. I've had to treat that many times when I kept angels - they seem particularly suscepible.

He seems to be back to normal - I'm leaning heavily towards him getting spooked and banging the lid as he is still reluctant to take food from the surface. Just seemed odd that this took so long to clear.

I had to put this big guy down yesterday...I had gotten him back to normal, but about 5-6 weeks ago, he began to have trouble controlling his body while swimming. He would do a somersault or roll over while swimming. Sometimes he would be unable to stop himself & cruise right into the glass. It never seemed to worsen, just stayed the same until I found him laying on the bottom yesterday.

I was thinking bacterial infection of the swim bladder, but I'm not sure if that is it or not. If that was the issue, I would think his symptoms would have steadily gotten worse. Anybody got any ideas? What should I do to treat this if I run into the same issue down the road w/ another fish?

I'm familiar w/ metro but never used kanaplex before...everything I saw lead me to think bacterial inf too, but it never worsened. Whole situation had me stumped - no changes in food, water chemistry...