We just went through a bout with them... I couldn't figure out what was wrong, because we read online that they like algae, but I try to keep the pool and or near the FC for our CYA level, but we realized that we hadn't swept the liner in several weeks, and there was some dirt and debris that had collected in a couple spots. We also figured we weren't diligent enough in keeping FC up ALL the TIME... like the BBB method says.
I swear I would put the net in at one spot and get 20-30 swimmers - I was shocked. Our process was threefold:
1) Diligently scooping them out and killing them over a couple nights (effectively halting reproduction!),
2) Sweeping the walls and floors, and vacuuming everywhere weekly at minimum (removing food source),
3) Keeping FC up at the target level for my CYA level - daily (poisoning their habitat)
(CYA= 50, so target FC is 6ppm; I would let it fall below that in the recent sunny days we had potentially caused the problem.)
And it has really gotten rid of the problem. I may find one bug once in a while, but it's so much better than it was, that I'm happy to scoop out one bug every few days. They are goners and the water looks sparkly!
Choots