I had the same setup with 160 gallon preformed pond and a
large submersible pond filter. I had two painted turtles
and 2 RES in it. I changed over 1/2 the water and cleaned
the filter about every 1 1/2 weeks. The problem I found
with the submersible filter is that it only contains a sponge
material to catch bigger waste. It keeps the water looking
clean but in reality it is not very clean. I now have
a canister filter and it works much better at actually cleaning
the water. It goes through several layers of filtration and
does a much better job. My cleanings are now about every 4-5 weeks. I think the fact that you are smelling the water means
you are not changing it enough and cleaning the filter enough.
Turtles are very dirty (much more than fish).

I would do water changes and filter cleaing much more often
with your setup to keep your turtles healthy.

Hope this helps
Linda

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