I agree 1000% that your alkalinity is way too low. Bring it up slowly - water changes are probably your best bet for getting it back into range especially if you are using a reef specific salt.
Because they are tied to the same water, I would tend to discount that as the reason that otherwise identical corals will do well in one tank and not the other. You say the lights are the same - but are they really? If T5 - are they the same age? If LED are they the same type and in either case, are they at the same distance from the water? Is the FT the same depth as the DT? You would be very surprised at just how much a little bit of water will attenuate the light, so if they are of different depths, really they shouldn't be at the same settings anyway.
Although they are separate systems, I have the same sort of issues with my DT (standard 4x2x2 120g) and FT (4 x 2 x 12" 60g) that started when I moved my old lights form the DT down to the FT (AI Sol blues) and replaced them with Radion XR30 G4 pros on the DT. I was lucky enough to have a friend bring over his par meter or otherwise I may never have figured it out. At what appeared to me to be the same intensity as the Sols, the par level in the DT was more than double. I quickly turned them down and allowed the corals to acclimate over a period of a couple months. Everybody in the DT is now much happier! The same thing happened to the FT - perhaps even worse. I had been using a pair of Kessil A360w's on the FT, leaning towards the bluer spectrum. When I put the Sols on it, I kept the same settings I had been using with them on the DT. Everything bleached and most of my frags died before I figured that one out. I reduced the intensity by over 50% and now finally things are turning around there.
What may look to be the same can be wildly different for a number of reasons. Just because you think they are the same does not mean that they really are!
Good luck, and keep us updated!!!
