Tried fertilzer this year, Aquatic Plant Food Tablets from PondCare, which are tablets that are 10-12-8 and you push them into the pot. Pretty nice but my pond's border is becoming a bit unstable so putting them in can be a bit of a chore. I put some in on 6-28 but I know I put some in eariler as well I want to say the beginning of June. So far the plants are doing better, and the algae is somewhat contained.
Today I put in a tablet in each pot. I currently have 3 water lillies. Two large pots and one smaller.