The air pump actually adds heat unless you are evaporating somewhere.
I try to keep nutrient between 68-74f, where the DO increases quickly.
More oxygen also deters pathogens.
You're going to have to keep track of lots of things in hydro including nutrient concentration and composition, Ph, temperature and etc. Write it down.
I don't know how many plants will feed from the 27 gal reservoir, but analyzing your data will give you a signal when things are going south.
You don't have to change the whole thing, but, if you don't, concentrations of things you don't want including pathogens will increase in the reservoir until you have problems:
Change the whole shebang: water's cheap. Once every two weeks is good.