I have a mulching question?

I have been there...4 years ago, I put a nice thick layer of mulch around my pond....the next 2 years, I just added a little bit to the top and it looked real nice again....then last year, I had these really rare mushrooms that would sprout up every other day, all around the pond. The really bad part is that this type of shroom attracted tiny ants like you wouldn't believe...I found out it was because the mulch on the bottom was mildewey and is the cause of these weird and annoying mushrooms....DO NOT JUST ADD TO LAST YEARS MULCH