I just added a bag of chemi-pure into a new tank full of fresh saltwater (no sand or rock yet). It's in a passive chamber in the sump that gets less flow than the rest of the system. I did it to theoretically make the water clearer before I add the "cooked" rock, and to see if it raises the pH, which was at 7.85.

Q: Are there any adverse consequences of adding chemi-pure, given that this was before the first cycle in this system?

I don't suppose that there are any negative consequences to the Chemi-pure but there is no advantage either. When did you measure the pH? It is low but it also falls when the lights are off. It will be hard to keep up if your house is really tight and the indoor CO2 level is high. Regardless, I would not worry about the pH until after your tank has cycled.

I don't see a problem either, but if there's nothing in the tank yet, you can just buffer it up without worry.

I haven't had my lights on at all yet. Yesterday, before the chemi-pure, my Pinpoint pH probe measured 7.84. Today, after 24 hours of passive chemi-pure, it tested 8.14. Thought that was interesting at least, but yes I concur with the information above. Thanks for your response.

Don't waste your money. There is no need to measure your pH while your tank is cycling. Let it do it's thing Measure ammonia, nitrite and nitrates only (and salinity if the water level drops). The simple organics of a cycle can drive it down... not that it makes a difference. Worry about the pH once it is 100% cycled.