Hello and welcome ,
Got him from a LPS or friend?
If a store chances are was not well taken of. They could have just gotten him in and have no idea when he ate last or what. Try a call and ask what they feed their turtles.
Have any red leaf lettuce in house , Any dandelion leaves from a yard , a little bite can tuna (kind in water not oil) He has to adjust to the new habitat and I would keep offering food he will talk when ready. Clean up what he does not eat. That's a small tank you may want the feed in separate container with some water to help keep main tank cleaner.
Can you answer these questions in a little detail it will help , (like "bulb" please give more than yes or no Type ,size and or wattage Please )
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