Does adding phosphate to pond water lower the oxygen level? Phosphate feeds algae growth. Too much algae can use up dissolved oxygen (the oxygen mixed into water that fish need to breathe). You fill eighteen jars with tap water and add algae to each one. Three jars get no phosphate and serve as the control. The rest get increasing amounts of a phosphate solution. You place all the jars under a grow light and measure dissolved oxygen each day. The control jars held the most oxygen. Jars with more phosphate drops generally had less dissolved oxygen. Even small amounts of added phosphate made a measurable difference.