How did you introduce him to the new tank? If they aren't acclimated slowly, the change in water chemistry can cause a shock to their system. This can result in symptoms like lethargy, or hiding.
There are many ways to do a water change and acclimate (introduce) your fish into the tank. You should always follow a procedure like this when moving your fish into a new tank, or after changing the water in his current tank.
Here is one variation of acclimation:
- Note the temperature of the water.
- Using a plastic cup, scoop him, along with some of his water, into the cup.
- Clean out the tank. Refill with water at the SAME temp. Be sure to add the correct amount of water conditioner.
- Float his cup in the tank for about 15 min.
- Add a SMALL amount of NEW water to the cup. (Several tablespoons, or about 1.5 ounces.)
- Let his cup float for about 10 min.
- If the cup starts to fill too much, remove a SMALL amount of water from the cup. Discard it.
- Repeat steps 5-7, until about an hour has passed.
(If this is a NEW fish, I recommend taking MORE than an hour before putting him into the tank the first time. Your water may be very different than the water he's used to. I usually acclimate new fish for several hours, just to be safe.)
- Gently release him into the tank.