What can happen with the water mold is it gets into places that are hard to get at - like the closed channels that you mentioned. You will have to take it up to shock, which for a cya of 45 would be 15, and keep it there consistantly for a couple of days to a week, or until the chlorine holds overnight. It sounds like you didn't kill all of it off, and it came back after you refilled. A shock of 20 isn't that high if you had really high cya. You also have to be consistant - keep taking the chlorine up to shock by testing it as often as you can and adding bleach. Consistancy and high chlorine will kill it off- make sure to keep the filter running 24/7 while you are trying to kill the mold. Feel free to ask any questions you have, and I do have sympathy
for you - but not for long, because you will win the battle of the mold
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