[Myka]

What you're describing is common phenomena. The cyano bloom happens in our tanks from GFO for the same reasons it happens with biopellets - quick, significant lowering of phosphate. In other words, too much GFO (or too aggressive with biopellets). It can happen with Lanthanum chloride (FozDown) too for the same reason. It doesn't happen in every tank though under what appears to us to be similar circumstances which is obviously frustrating. Just as frustrating as high nutrient SPS tanks that get no waterchanges or dosing and grow SPS like weeds for years and years (lol). Anyway, the reason the cyano appears is because the water column is suddenly barren of phosphate yet the substrate and rock is still loaded. As the phosphate leeches out of the substrate and rock the cyano jumps in to feed off it as it leeches. Cyanobacteria are extremely efficient at processing phosphate.

The fix: Pull the GFO. Keep using carbon. Add a filter sock, then use a piece of rigid airline tubing with flexible airline tubing attached to make a mini siphon hose and suck the cyano out of the tank at the peak growth of the day (usually at peak lighting). Do this daily or every second day. After you suck out as much as possible use a turkey baster to blast every nook and cranny in the rock. Then use a feeding stick or the baster to stir up the top 1/2" of sand. Wait an hour and remove/change out the filter sock. It takes elbow grease, but it's very effective.

Next time, don't use so much GFO. 1 tbsp per 30 gallons changed out every 2-3 weeks is much "safer" than adding a whack and changing it out every 6-8 weeks.

Added note... in my experience, mature tanks shouldn't need GFO, and usually do better without it.