What's the best way to add 18 inches of height to a 3 foot pond made out of concrete block
I would think two more rows of block with a 2" flat cap would work if you can top the existing blocks. Something to consider; do you want the added height or do you want deeper water? It might be just as easy to dig it down and leave the existing walls where they are. It would depend on what your base is, I imagine. Good luck.