Hey man. We actually started almost at the same time. I started germ 3 days after you! This is my first grow so i'm a noob. Just remember that. I'm learning just like you. One thing i'm definitely learning is that hydro ain't easy. Not by a long stretch. Everything's got to be under control and things can fail very quickly.
I don't know anything about your nutes and mine are completely different, but do you know if you're using any type of CalMag (Calcium / Magnesium) product? I had a similar problem with climbing pH levels too. I found out by reading other threads that low ppm water (like yours) lacks any good kind of pH buffer and that CalMag can help smooth things out. Luckily, my nutrients kit came with something called MagiCal and with just a little of it, my pH has been pretty stable since i started using it. The most it'll go up in a day is like 0.3 now. Funny. Neither of the two grow books i bought covered this scenario.
The pH of your solution is supposed to change over time because your plants crap out higher pH crap than what they eat, water evaporation, etc. And remember, the pH of your medium is every bit, if not more important, than the pH of your soup. You need to make sure nutrient salts don't build up to deadly levels in your growing medium.
As for your reservoir, i think it's the heat down there. Have you been monitoring reservoir temps? I don't know what that is down there, but it just looks like some kind of rot to me. Doesn't look good. And it doesn't look like other reservoir pics i've seen anyway. Even if you don't have light leaks, i think the heat alone is enough for it to be a serious issue.
In the pics of your new setup, what are those clear tubes? From what i've read about hydro so far, clear tubes are a no-no. They are just another light leak.
Question: Why did you put your air stone under rocks? Why do they need to help mix nutes when the nutes should be mixed already? I just ask because i haven't seen it in other grows and i thought the sole purpose of an air stone is to oxygenate the water. Hell, i'm terrified to let anything into my reservoir without a super sound reason, hehe. Btw, when's the last time you changed reservoir? What's your reservoir change schedule?
Answer to your question: Yes, the reservoir needs to be oxygenated 24/7. Turning off the pump is bad.
The good news: It looks like your plants are still chugging along so that's good. I hope you get your pH and reservoir issues get fixed.
The bad news: I think you have 2 main obstacles. One is heat. And the second is stagnant air. When i imagine your setup, i imagine an oven. A hot box without a way to get fresh air in. Even if you have a fan in there, no fresh air circulates in, and you're just recirculating "used-up," Co2 depleted air.
It might not be a bad idea to take a step back and reassess. You might get over the pH and reservoir issue for now, but down the road, you're going to face worse problems. At least now, they're only seedlings and you can deal. But you don't want shit hitting the fan when your girls are big, bushy, sticky and smelly. Shutting down your grow temporarily may not be a bad idea. First, create the conditions to grow right. Then
grow. You've come this far already. Might as well take the last few steps and figure out ways to solve heat and air circulation. And then, after that, only your carelessness can stop you from that big, stanky, dank stash we're both workin' for. Best of luck!
Hey, it just occurred to me. You said you had no air circulation. How are you going to deal with odor?