Or is there some other way to attack this problem.

The following worked very well for me, so you might give it a try.

Drain your pond until the area that needs sealing is dry.

Get RIBBON CAULKING (the flat kind that goes around doors when a new
door is being installed) and use that to seal whatever/wherever the
water leaks from.

Plumbers Putty might do the same thing (ribbon caulking is all I had
at the time I needed to seal a pipe in my filter)