+1 on the Mg and dKh being too low, IME, dKh bottoms out and becomes limiting at ~6. You MIGHT be adding just enough dKh to your small tank to keep up, but not enough to your big tank. If this were the case, a small increase to your dose, perhaps 20%, will start to raise your dKh on your small tank, but not your large one. You would have to substantially increase your dose on your large tank (at one point I had to triple my daily dose to start raising it) to see an increase. To test this, and ultimately restore your dKh to appropriate levels, I would increase your dKh dose by 20% or so on each of your tanks every time you test it until you see it start to raise, then when it hits 9 or 10, cut your dose back to maintain that level (ie week 1 dose 1.2x, week 2 dose 1.5x original dose, week 3 dose 2x original dose etc).
You may also see an increase in consumption associated with installation of your dosing pump as a result of increased stability, so may have to increase again. Of course your coral issues in the larger tank may be related to other things as well, but the low dKh, IMO, is at least contributing. The same would go for Mg, but I have found it does not become limiting until well less than 1000ppm, although it can slow growth down and potentially mess with your Ca as you start to fall below ~1200ish.
Good luck and let us know how it goes
Dan