Does pH change when covered/with lack of sunlight?

[chem geek]

So, the conundrum continues. I guess I should add some Borax?

Yes, you can use 20 Mule Team Borax to raise the pH. If the TA were very low as well, then you could use Arm & Hammer Super Washing Soda (careful: NOT the laundry detergent) instead or some of each depending on how much the TA needs to be raised.