I am no expert, but I believe 15-18 years of build up is a problem, but definitely not too much to overcome to make your pond a good fishery again. The only way to eat an elephant is one small bite at a time!

Do you have a stretch of shoreline that used to be one of the good fishing spots? If so, you might start there by killing or clearing the cattails for access, and then killing off some of the watershield to make a fishable area.

I like starting small on new projects on my farm. Even with expert advice, I almost always discover that I have a learning curve and can quickly get better at the new skill.

Hopefully it will be the same for you when attacking the watershield. A small kill will be safer for the pond, and you can tackle bigger "bites" in the future.

Good luck on your pond rehab project!