haha dont supposed a hrv is out of the question
im guessing the swing is because this tank is in your bedroom and theres not a whole lot of fresh air coming in i think during summer youll find you wont be having these swings as drastic. is it a reeftank you have?? if so unless theres any drastic change in your corals appearance i wouldnt worry about my ph to much i would just get my alk back to where its supposed to be once that stays consistent i think youll find your ph will balance out.