It sounds like your fish was OK until you moved him into the new larger tank. Is that correct?
For now, can you move him back into the smaller tank?
Here is what I'm thinking:
1) Transfer him back to the small tank.
- If he is OK in the smaller tank, it's likely that the problem is something with the big tank. This could be water quality, the new filter, a new decoration, etc.
- If he still has problems after going back into the smaller tank, then the problem is fish-related (not tank related). And we can use his symptoms to determine what's wrong with him.
Follow whatever procedure you normally use when you do your regular water change, and put him back into the small tank.
Let us know if this helps him recover.
2) Remove all the water and everything new from the big tank.
Since your tank is still new, I would just dump out all the water and start over right now.
Also, whatever is new in the big tank, remove it. It's possible that something new is bothering him. For example, some decorations have paints that pollute the water. This can be irritating for the fish. For now, I would remove anything that is new. Just put it aside for now.
3) Refill the tank with new water. Only use decorations that were in his old tank. Don't use anything new yet.
4) Get either the SERA TOXIVEC or TETRA AQUA SAFE water conditioner. Add it to the large tank.
Sera Bio Nitrivec - the product description says it: "Breaks down ammonium and nitrite due to the included purification bacteria...."
But ammonium (NH4+) isn't as much of an issue as ammonia (NH3). And this product won't neutralize chlorine/chloramines.... Ammonia, chlorine and chloramines can all be toxic to fish.
I would get the the Sera Toxivec
or Tetra Aqua Safe
conditioners. Both neutralize/remove chlorine, chloramines, and heavy metals.
also neutralizes ammonia, so if possible, I would get this one.
5) If your fish is now feeling better, move him into the big tank.
To do this:
1) Add a small amount of water from the large tank into his small tank.
2) Wait about 15 minutes.
3) Repeat the above two steps 3 more times, until an hour has passed.
4) Gently release him into the big tank.
6) Watch him closely. See how he is doing.
If he is OK, add the filter back in. Watch him for 2 days to see if he is OK.
If he is with the filter, then add any new decorations - ONE AT A TIME. Wait at least 2 days after adding each item.
If he looks like he isn't feeling well after a new item is added, remove the item and do a partial water change.