There’s far more to the equation than that. For one, your tap water alone has enough trace phosphates in it for algae and plants to be competitive with bacteria. Secondly, your phosphates are not very low (IE in the order of 0.001ppm as it is many ULNS or those using chemical phosphate removers). Thirdly, food is not your only source of phosphates.The only phosphate in my tanks is from fish foods. If low phosphate encourages cyano, my tanks should be smothered.