Hello dcurzon,
You can easily add a dark sediment to the top of the sand, or you can gradually take out and replace small sections at a time once a week or so.
GDA is also CO2 related and is also considered to be one of those "life cycle" types that after a while will go away and become dormant. Very unpredictable. The conventional wisdom is to leave it alone for 3-4 weeks and then remove it all when doing a massive water change. Sometimes it requires a second 3-4 week growth/removal.
The pH drop should be 1 unit, so as mentioned, try to turn the gas on earlier.
Cheers,