Sometimes moving into larger tanks can be stressful on them - outside of the grow out tanks, they are in small spaces. So going into a bigger tank they may need some time to adjust.
You definitely want to get some plants and a cave for him - for both mental stimulation and to have the feeling of safety. A larger tank with no where to hide or nothing to rest on, nor anything to explore will have him just hanging around in one spot. Not very fun for you nor the fish.
Look to see if there is a valve to change the flow of the filter, if not you can use an aquarium sponge cut to fit into the outflow section of the filter to help reduce the current.
Would move the heater down and sideways, don't want the water to evaporate and expose the heater - a good way to get it to malfunction or even explode.
Or you can raise the water level close to the top and then you can leave the heater there.. if you have no hood then about 2-3 inches from the top is fine for the water level. No point in having a 5g with only 3g of water in it :)
You can purchase plastic crafting mesh from Walmart for cheap and use that as a lid - will allow light to come in, and you can raise the water level to the top.
Once he has plenty of plants and a cave he should become active once more.
Last edited by Myates; 12-21-2012 at 08:31 AM.