Ok so just finished filling a brand new plaster pool (diamond brite exposed aggregate finish)this morning. We did an acid wash before filling then added a ton of acid after filling at the direction of the plasterer. Like 12 gal in a 16k gal pool.

Testing ph, it's lower than 6.2 using the TFP kit. (YELLOW!) and testing TA the solution turned red before I even added any of the 2nd reagent. What does the latter mean?

Thanks for the hand holding.

12 gallons of 31.45 % acid in 16,000 gallons of water will lower the TA by about 380 ppm. So, subtract 380 ppm from your fill water TA to get your current TA, which is probably strongly negative unless the TA was above 380 to begin with.

The pH will be well below 4.5.

This is called an acid startup. However, 12 gallons of acid in 16,000 gallons of water is pretty aggressive unless the acid is a lot lower strength than 31.45 %.

The startup period is one of the most critical periods of the plaster's life. Be sure to follow the plasterer's instructions and follow-up with them to make sure that the process is completed properly.