I have a 45 gallon tank, with 72 watts of AHS light. I dose 1/2 tsp KNO3 and 1/4 tsp or a bit less of KH2PO4 every other day, with 1/4 tsp or a bit less of CSM+B on the other days. So, I suspect you are underfertilizing by quite a bit. My experience was that I needed as much plant mass as I could get for longer than 6 weeks, too.

The major difference between our tanks is that I use pressurized CO2 instead of Excel, so my growth rates are probably a lot greater than yours. Considering that your fertilizing may be good. And, I switched from 96 watts to 72 watts of lighting to reduce my growth rates a bit.