Phosphate from Dead Algae

[tackful]

I recently used ethromycin to treat a somewhat modest green algae problem. It was showing up on older leaves and in small places on the glass. I removed as much as I could, especially from the plants, then did a 30% water change and tested for phosphate. The result maxed out on my test kit, whereas my normal level is around 0.5. Was this caused by the dying algae?

[tanker]

Could be. Dying plants release their "stored" nutrients back into the water.