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Goncharkina, if you have algae growth, you are getting a false phosphate negative. The algae uptakes the phosphate before you can measure it. Unless you are using a photometer such as hannah checker's phosphate or phosphorous, the low readings don't mean much. API and such are notorious for having poor low end readings.
You'd need to verify you follow the standard procedures for low phosphates. IE rinsing food, RO/DI water, etc. Lighting period can also affect it. Are you running an ATS (algae turf scrubber) or gfo?
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