[orcafood]

Makes sense to me, outside is your headspace. I still recommend the design should bubble air through the solution being tested for pH rather than rely on diffusion.

Should work with a serious air injection system on a tank, such as yours. Otherwise the headspace might need to be used.

Ideally we need to know the pH of the solution to at least the thousands place, which you are not going to get with a meter, not to mention the pH meter will drift.