Wow, it’s amazing, I got lucky and found a video that covers exactly what you asked about testing the run-off, and then he talks about heat.
Check it out:
Then some other guy talking about just testing pH in the run-off as well. I agree, pH in the run-off can be a little lower than what you put in without it being a problem, but it is best not to let it it get too far out of range.
~MacG