Instead of a Shop Vac for bottom sludge removal -

I use a 12 ft. length of clear 1" plastic tubing to siphon off the
gunk on the bottom of the pond every spring. I taped about 3 ft. of
one end of the tubing to a length of PVC pipe, just to make it easier
to handle when jamming that end down between the rocks on the bottom
to suck up the suldge. The clear tubing works well, so I can watch
what's getting into the tube. Once in a while, I'll have to rescue a
goldfish, but they survive the ride OK.
Hope this is a helpful idea for you.
Gary