Arduino + PH probe in Aquarium. Ground Loop. Please help.

Did anybody ever get this to work with this diagram?
Doesn't seem to work for me.

My next step was to remove the optoisolators and just try the DC-DC converter itself.
That didn't work either and I think the reason is that there's supposed to be an inductor/resistor combination on the output side as per the murata datasheet p4, but I don't really know.

Thanks for any help.