I am not sure if its HO or NO... In the spec sheet, its says T5HE. What are the differences between these?
So, I hope changing actinic bulb with equal 21W or possibly little higher 30W or so should work just fine?
T5HE might be "high efficiency", meaning you would have T5HO (high output) lighting. This is in contrast to T5NO (normal output) lighting. As the name would imply, high output lighting puts out more light.
If you change out your actinic bulb, I would just get a similar rated wattage bulb to replace it; your ballast may not be able to drive a higher wattage bulb.
With 42W of T5HO lighting, you would have quite high light; be sure to keep up with your fertilization regimen and CO2 to ensure healthy and algae free plants.