Hey have you been testing the tank water? Some NPTs, like when I set mine up, have period where the plants aren't actively growing so there is excess everything in the water, including ammonia. Mine went up to 4 ppm, which is lethal, so test 1 a day at the same time for ammonia, you can do nitrates, but ammonia is a much bigger killer. If it starts to increase get him out of the water until it returns to normal. One way to help this is if you add a bit of your filter media from your bigger tank. That will have some bacteria and they will convert ammonia to nitrate if there is a need.
These tanks always look okay at the start, but after, they turn it great tanks. The one in my picture is an NPT as well.
15 Gallon NPT
1g no tech bowl in the making