Wish I had me an Apex, that is probably why I don't focus on integrating to one. I think the liquid needs to be water, what else would you use? Many other liquids are volatile and would end up in the tank water eventually. Agar has large pores but I don't think you want to slow the diffusion down through reference solution, I think quite the opposite we want air vigorously pumping through, like your air injectors, keeping the headspace equilibrium. pH meters are expensive for nice ones, like $100 for a good probe with longevity. Green laser diode $5, dye free essentially, photosensor $2-5, half mirror $10, I'm sure there will be hidden costs. Think hanna phosphate checker. Total cost of around $30 tops and likely better resolution than could be had with a pH probe.
If the reference solution is isotonic with the other liquid (tank water) in contact through the headspace there shouldn't be evaporation. The headspace will be saturated with humidity.
I'm sure a pH meter would work just fine
