Carl,
It's not the water as I use/used nothing but UCSB NSW in both tanks. Matter of fact, they were both started with NSW and even my new nano was started with NSW.
In the 72G with NSW, I had a green w/ purple edge monti that grew like a weed. I think everyone in the club got a small colony from that and it was still huge. I also had a large assortment of zoa's along with some LE SPS corals. All doing good and thriving in the 72. But they didn't survive the move to the 400. I even had a large Superman monti that I found at the LFS. I split it and put half in each tank. It grew good in the 72 with nice colors but died in the 400.
About the only difference between the tanks besides fish load is the sand, flow and my maintenance. The 400 has Hawaiian beach sand that's been in there for years. The 72 I started with new store bought live sand. The rock in the 72 came out of the 400. I have a whole lot more flow in the 400 than the 72.
My calicum reactor on the 400 broke and was out for awhile. On the 72, I used Seahem's Reef Builder and Alakinity. Reef builder has calicum with some otehr goodies in it like magn and stroninum. Don't know if that was the difference or not.
Also in the 72, my coraline alage grew like a weed but I can't get it to grow in the 400.
I'm now working on trying to get the 400 up and running good so I can try some corals again. But it might be I have too big of a fish load for that. AS to what happened, I don't know. I had a 6 + year old pagoda coral that looke dgreat and was doing great in the 400 for a couple of years. When I added the new corals, it went down hill and I couldn't save it. About the only corals I have left in the tank is my large toadstool leather that's hanging in there, my two pearl bubbles, one of which had spawned in my 45 years ago and is not just barley holding on, and then I have a blue ridge SPS coral that's still growing.
As for water paramiters, who know, I haven't tested them in years. Last time I tested was just for calicum and magnisum and that was just a tad low. That was about 6 months ago and when I was trying to get my calicum reactor back up and running. I've also been adding kalkwasser to the tank on a drip system but don't know how much it really has been helping. But I'm still plugging away at it.
I'm trying to get back on a regular water change schedule. For about a year, I was just doing 50% changes about once a month. I'm trying to get into a 10% a week on water changes now to see if it helps any. I did do a couple of 75% water changes about a month ago. One because I did a gravel vac and the other because my out side filter broke and pumped out about that much water before I could get it secured.
But I'll keep plugging away at it. Just been really busy lately trying to get my bike painted so I can ride again. That's taking up a lot of time since I'm doing it all myself.