Nitro

Merry Christmas and welcome to the forum. First you shouldn't be using a floater with tabs (Trichlor) in a hot tub. The hot water will make them dissolve too quickly raising the FC. And they are too acidic which will drop TA/pH. Both of which you are seeing. I'd recommend reading the top link below for the correct way to use Chlorine.

Two things. First, your CD is actually higher than 70%, because you didn't wait ~24 hours to test the second time. Second, just because your CD is normal doesn't mean your tub is free of biofilm, which can cause problems. I recommend doing a Decon (bringing FC up to 100 ppm) regardess.

The Pool Calc will do it for you. It takes approx 1 oz Dichlor to raise FC 10 ppm in a 400 gal tub. A CH over 100 is ok, just make note of it. Bring your TA down to 80 ppm for the first week while using Dichlor. After you build up 30 ppm CYA, switch to using bleach as your sanitizer, and lower TA to ~60 ppm. Read my top link below for full details.

Yes I did. Thanks for the correction.

NewB, IMHO your tub is not safe, because you've let FC drop to zero for too long. You have a VERY high Cholrine Demand, so that makes it even harder to keep FC up. I seriously recommend doing a Decon, preferably with Spa flush. However if you don't want to (can't) change your water twice you can just do this. Add 1-2 Cups MPS, then 1 Gallon of bleach. Let jets/air run for an hour, and let sit for a few more. Wipe down spa (and cover) with super chlorinated water. Check FC. At this point I recommend refilling. However, if you don't want to refill yet, you can add Hydrogen Peroxide to lower your FC. It will take approx 1 cup to lower your FC by 10 ppm. I would lower FC to around 50 ppm if not already there, then let sit overnight. Check FC again to see what your CD is. If CD is 50% or below, you're good to go. Then do a refill ASAP.

Keep in mind if this is a new tub, you will want to change the water after a month or two anyway. After this first refill, the water should last ~6 months. You can use the Pool Calculator to calculate how much chems you need based on tub size. It your case (400 gal), you should add an ounce of Dichlor. It doesn't have to be exact for calculating CD. Just measure it and make note. I use, and recommend the Taylor K-2006 drop test kit. I don't have experience with electronic testers.

TA won't rise unless you add something to make it rise (i.e. Baking soda). Bleach alone should not have effected your TA. I believe there must have been an error in testing. I suggest you measure TA/pH again, and make sure you get an accurate reading. I assume you're using a drop test kit, and not test strips. Then proceed from there. i.e. Lower TA as needed.

I think we are pretty much saying the same thing. I just like to track it so I get a handle on where it needs to be. BTW, carbonate alkalinity may need to go as low as 40 to gain pH stability.

So if the ph is 8.3 and I add acid and the ph goes up again I add more acid. I keep doing this until I the ph stays in the midrange, ignoring how low the alk becomes? I'm thinking it will be very low if I do this? Yeah, but it should happen with TA around 50 ppm. As Quant said, add Borates to slow pH from rising.

You can adjust CH before or after. Unless it's much lower than 100ppm, I wouldn't touch it. Just make note. Using the tub's jets/air should raise pH if TA is too high, so I'm not sure the need to use BORAX, other than to add Borates during startup.

Guess I'm the slow one here. LOL

You don't! Instead, adjust TA until pH is stable, below ~7.8. That means you need to lower your TA to ~50 ppm if needed.

This is where we disagree. I prefer to speed up the process by lowering TA until pH stops rising. This can be done in a few hours. The reason is, some people have very high TA levels (300-400). In which case it will take them a long time (if ever) to get their pH stable, without paying attention to TA. Therefore, I find it's better (easier, less trouble) to adjust TA (to 60-80) from the get go, and fine tune over the course of the next few days, weeks.

I totally agree! You have nothing to lose, and I nice clean, safe tub to gain.

I recommend you Decontaminate first (see below). Then follow the Dichlor/Bleach method. Use Dichlor of a week or two to build CYA to 20-30 ppm, then switch to bleach. (see bolw) I'm not familar with electronic testers. I would consider getting a good drop test kit. Taylor K-2006