While madwil's technically not wrong, I would disagree with using Muriatic Acid to lower pH. Personally, in your case, I wouldn't do it. But it's your choice.

7.8 is a tad highish, but not seriously so and still a swimable level. Your Tri-Chlor tabs are super-acid and will, within a few days, bring that pH right down. Since you are using them anyway, let them do the work and don't risk over-shooting your pH goal. It won't take the Tri-Chlor long to bring it down to 7.6.

Your Alk is very high but I'm assuming your calcium level is very, very low, so it's not a serious problem.

You COULD add MA to bring your pH and alk down, then aerate to get the pH back up, and that's not wrong, but I prefer the more conservative approach:

Let the Tri-Chlor tabs bring the pH down slowly, then aerate your water to raise pH back up. This will gradually bring Alk down with it. Just do not use ANYTHING containing calcium.

Carl