I'm sure that you have read about a sechi disk. This is a good indicator of what to do. Doing both is right but every pond/lake is different and will respond differently as well. As you start out doing both - monitor your bloom and once you reach 18" in the Spring and Fall and 24" in the summer - back off either both or more probable the fertilizer. Listen to what your water is telling you - your hearing aid is the disk.