T5 lights.

Is there T8 lights that are high output? Since mine is just 15 watt, is there T8 lights that are 24 watt?

T8 lights are normally fluorescent light tube and there're no high output like those of T5. There is a 25W T8 tube available.

However, there are T5 llights just for freshwater fishes available and the cost is about $45 per tube depending on the tube length.