hi
please make sure you got someone with you when you go into the pond incase of any problems

i had to get into my pond 2 years ago upto me knackers

in january

that was cold.i had to glue the pipe returns to the liner
and was in the pond about 6 or 7 mins my legs started to go numb and only just had the strengh to climb out of the pond as pond is 5ft deep
nearly had to call my son to drag me out.
you could try and turn pump around and pump out through bottom drain see if that moves blockage

just a thought before you get your speedos on

good luck

Dermot