As mentioned, RO is good but the resins in DI will eliminate compounds which are suspended in a liquid form. Phosphates were mentioned, silicates are another pottential nutrient for the nucience algae. I've been there and replacing the DI resins made the difference. It's a small system which makes it very unstable! Change your Source water, try a different salt too. Keep it simple and stick to the basics. It's a crappy battle, I think we've all been there.
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Doug
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