Do you test your water for No3&phos sometimes The wrong ratio of the two can cause this, also you could try increasing the circulation in your system A stronger flow the better and with the extra flow keep siphoning off as much as possible The reason for the extra flow is because it's possible that nutrients have just built up in low flow areas ,these are just a couple of possibilities worth trying, as in my opinion I don't believe turning off the lights for a period of time really works in the long run ,because you really haven't solved the problem of what's causing it in the first place and will end up coming back sooner or later.
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