Doh!

a simple U shape and a PCV or acrylic tubing bent in a U shape with a cap on one end.


IIIIII
III III
III III

The capped end would have the ref solution, the other open to the tank.

You'd just leave an air gap on the top of the U.
You do not need to see the solution other than to make sure it's there etc and add a pH probe to this.

They already make pH in line probe assemblies.

A 1" PCV elbow(the one's with the flat end, not curved) and a grommet would likely work well.

Regards,
Tom Barr