Well, GDA is easy to eliminate.
It's appears when there is an imbalance related to Po4 and Ca.
If Ca is high this is because you probably had added too much. 50% water change solves it. And reduced the quantity next time.
About Po4, Water change helps if the new watter hasn't Po4. Reducing the ppms you add helps a lot. I think that 0.5 is enough unless you have a lot of microsorums, marsilea crenata, etc.
This two items should be in your mind in order to avoid a new GDA problem.
Finally, to eliminate it you should use the "generic protocol of the Kno3". Stop fertilizing at all. Change 50% of the water. Add every day for 1 week 1 gram of Kno3 every 50 gallons until GSA appears. If after one week the GPA doesn't appears, then repeat it but 2 gram every 50 gallons and so on.
I know that in this forum you guys have very different ideas about how to deal with GDA. This way to solve it was sucesfully used hundred of times during the last three years.
You can find more info about this in this thread / article:
http://www.drpez.net/panel/showthread.php?t=154436
You know, when a theory doesn't work, then this theory should be abandoned.
Method of controlled imbalances and gda... 
Threaded Mode
