Odor detected today

I check my pond temp every other day or so. It is staying in the 70's
since i started partially covering it with cardboard during the day. I
can see nearly to the bottom, water looks clear. I have loads of
plants and a small fish load. Today i pulled up the thermometer and
there was a terrible smell. I didn't think the thermometer reached the
bottom, but i could be wrong. I haven't done a water change in a while
and will do that tomorrow, but i suppose i am supposed to be vacuuming
the pond? I planned to do this this fall anyway, but should i do it
now instead or even also? Is this a major red flag and i should act
now? I have only goldfish and a 70 gallon pondlet. No filtration, etc.
I am also afraid of killing the snails i have in there. I think i am
going to set up an aquarium and syphon the vacuumed water through a
net and get all the snails i can. Think they will live off of
anacharis and algea til next spring?