The culprit, to me, is pretty obvious: You took 3 frags that were stressed from shipping, and put them into a 1 month old tank with low temp and high alk/calc, which further stressed them. Can you check with the seller to see what the tank params of the frag system is? I'm guessing their temp is around 78, alk around 8-9 and calc around 420.
I am not sure how the age of the tank has anything to do with how well the frags adapt to it though.
The temperature, I would imagine would not play a key role either, given the frags had to likely deal with varying temperatures during shipping, so acclimatizing to 75.74 F or 78 F probably should be similar.
But I can see the problems with high alk/calc. I would definitely imagine though that the alk/calc from the seller would be way lower. Though I did have another three coral frags I bought from the seller that was added to my other tank that are doing great. Of course, that may not really say much given 1. they are softies and 2. the other tank's salinity is 1.023 sg and probably had lower alk/calc.
Maybe, maybe not, but also not helping the situation.
Here’s what I would do with a tank of that size... Remove frags and place in a small bowl, using tank water. Do a large/full water change. Get parameters in check using correct salt, or self adjusted. Acclimate frags to tanks new parameters. Reintroduce frags to tank.
Yep, I am doing a water change now to get parameters correct.
Expensive lesson learnt, the frags have 'stabilized' now in the other tank, but if they do recover it seems like it will take a very long time.
Gosh I feel bad. I figured the alk/calc was high, but forgot to take into mind that going from the seller's tank makes the change even more significant.
Thank you
@T-J and
@DaneGer21 for your recommendations. For what it's worth, I have also changed the temperature to be higher, and yeah gonna see what the parameters of the tank are before considering re-introduction of the frags.