the only solution that I can think of (without digging out a part of your concrete base with an angle grinder and a Kango hammer ...this is "do-able" and has been done before!)is to install your bottom drain and pipe work on the current base, cutting a hole for the 4" pipe in the pond wall, and then pour concrete over this.
This solution will be perfectly OK but your pond will be 6" less deep.
Hope that helps ?
Posted on Mar 21, 2006, 4:11 PM from IP address 86.142.11.224