It's usually started by too much waste, not enough filtration. Goldfish are very messy fish. I would get a bigger filter. Also, do you test for ammonia, nitrITEs, & nitrATEs. I suspect the nitrATEs are high.
The best way to get rid of them is to starve them (food & light). Perform a gravel vac and 30% water change. Then cover the tank COMPLETELY using a dark garbage bag & dark blankets. There must be NO LIGHT getting in. Keep it covered for about 2 days. Fish can survive almost a whole week without feeding. After the few days, preferably during the night, feed your fish lightly. Repeat this until the bloom clears up. I'm afraid this may occurr again UNTIL you replace your filter with a bigger or at least add another one.
