Yep it will increase naturally. The lower your holding capacity the faster it raises. Mine goes from 5.5 to 6.0 over 4 days time. When I was in DWC it would do that in 12hrs. I have 30 gallons for one plant atm so yea. . pretty mild swings. As for your PPM, you want to slowly watch it drop. So when you come back and take your ppm check in 12hrs, your ppm should have dropped from 296 to like 280, and so on. This means your plant is eating. Now if you come back in 12hrs and your ppms have climbed, say to 320ppms, then your overfeeding and it wants more water. Dont overfeed, it slows the growth rates down and just hurts all over.
So the issue comes with heat. I have always put in a exhaust fan that is too large, one that I could never run on full power. Now to remove heat I just turn it up. So when I turned up my lights, the heat in tent raised, and then i just exhausted it, or you can sealed room ac/duhue it, either way dont use your light to manage heat. You must always increase your lights, if your tent gets to hott, you have to put another fan in it. . dont turn your light down to manage heat, your growth will just slow down for no reason.