I use to use Excel on my 55g with good results and I dosed 2 caps daily and after large water change I'd dose the 5 caps.
Try increasing Excel like mentioned.
Well re-reading your first post you have the Glo fixtures right, which is T5HO and you have 4 bulbs. You should have plenty of light and its the carbon source that's being short most likely. You are probably in the moderate/highlight range. I'm not the best with light determining. Lol
All you can do is make small adjustments and observe.
You may want to pickup Potassium as well and Iron.
Switching to dry fertilizers would be even better.
Last edited by AlyeskaGirl; 11-26-2012 at 04:04 PM.
Reason: Added more thoughts