I'm using a tiny Tetra 10 air pump which is very quiet and provides enough air circulation. I can't hear it from a few feet away, last thing I want is to hear bubbles from my plants while watching a movie.
My PH is 6.0 and once it creeps up to 6.5-7.0 then I change the solution. I haven't had to change it yet in the keg, it's been about two weeks so far, the keg holds 4.5L of nutrients. As for EC, I don't have a meter, I mix it as per the nutrient bottles. EC doesn't tell you which nutrients are exhausted or which there is an overabundance. I go by PH and change the solution when it creeps up.