Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Add a complete fertilizer, including N, P and K.
Comprehensive (I think) has low to no macros, it is more of a micros fert.
Here is the maximum dose of nitrogen you can give a tank:
Cycle the tank by adding ammonia, but not higher than about 1 ppm. Many plants are OK a bit higher (some are OK a lot higher), but many are not. 1 ppm will grow plenty of bacteria, though. You can test and bring the level up to 1 ppm twice a day.
You can dose P in a ratio with the N. Something like 1 part P to 10 parts N, though the exact ratio is not that critical. So whatever source of P you are using, you will have to do some math to figure it out.
K can be dosed a lot, too. Easily equal to the amount of N.
The nitrifying bacteria will be using the ammonia and carbonates, so keep these up. They probably use a very small amount of the trace minerals, so whatever dose is the maximum on the label, use that.
What are the actual wavelengths of the bulbs you are using?
The K value is a sort of average of how our eyes see the light. Not really applicable to plants. Get an accurate PAR reading for those bulbs. You might invest in a PAR meter, or join a club that has one available and borrow it.