If you have not yet fertilized your water lilies, now is the time. Use aquatic plant tabs and push them into the soil. Use one tab for each gallon of soil. If your lily pads have not yet reached the surface, wait until they do.
I do not fertilize other aquatics. The fish waste seems to do a fine job of that. The plants leap from their pots as it is. With fertilizer, they would be camping on my front porch.