50% water change and still 20ppm nitrates

[o.c.d.]

You said your sure it nitrate because of the algae growth. Frequent water changes should lower the nitrate if not your kit is probably bunk. Have you tested the po4? Cause it's the algae that is also making you mad right. So if you stopped using GFO your po4 may have started accumulating over what the zeo bacteria can handle consuming. Over time the po4 can accumulate then high enough it will precipitate and bind to the rock in solid form. Then slowly dissolves esp when the O2 drops, That then provides a perfect food source for algae, water change will do little for that situation. Also po4 in a solid form will not show up on a test kit. It then becomes a tedious process of removing the po4