Man, I couldn't agree more, I had a 20g sump under my (about) 90 gallon tank and I've been carbon dosing to maintain 4ppm nitrates.. everything looked amazing. I relocated my new sump to the basement 75g tank and a 30g frag tank which was new.. the first week was great for my display. Corals looked amazing water looked clearer. But the frag tank definitly went though all the uglies and still is, dinos, diatoms, sprouts of hair algae. My nutrients bottomed out and I've lost 6 acro's in the last week. Now I stopped the vodka and have to dose nitrates and phosphates to try to stop the problem. I kinda wished I cycled the frag tank with the sump for a few months then tied in the main display.If you folks have not seen @Tuffloud1 build thread do so. It is awesome!
A couple thoughts/prognostications:
a) Should not be a cycle. Will be a very typical, dry setup, uglies phase.
b) Biggest risk is to the 90G acros struggling with nutrient deficiency. Big risk IMO.
c) Next biggest risk is dinoflagellates, but they will only inhabit the new tank, not the old one. Still, they will strip the nutrient out of the water, contributing to risk # b.
If any of you have spent as much time in the Dino thread as me, you've likely watched me go through this scenario. Three times. Every. Single. Time that I added a tank to an existing system it broke out with dinos (ostreopsis thankfully).
Here is what I would do:
a) Plumb it for the long term. Absurd not to connect these two gorgeous tanks.
b) Plumb it for the short term also (with isolating gate valves. Sorry, extra work.) **edit** I see u planned already for this
c) Run them as separate systems, but transfer over some rock, sand, whatever media you can spare to the sterile tank.
d) I don't know your stance on WCs, but once a week I would open the valves to run them as combined for, say an hour.
e) Feed the living daylights out of your 90. You just did a 100% WC. (I would skip Aminos as dinos crave them.)
f) Load up both systems with fish as fast as is safe. Dose bac if you have to on the new tank.
g) Buy buckets of NO3 and PO4 and set up your dosers beforehand. Keep careful measures of nutrients and ALK.
h) Be prepared to apply UV to the affected display tank temporarily.
i) Dry skim. No GFO, no carbon dosing. Run it so dirty your Mom would be disappointed in you.
Points G and H are kinda tricky as to where in my scenario they should be installed. Probably the UV should be on the "new" and the NO3 and PO4 dosing on the "old" where your acros are.
Apologise if this reads bossypants or know-it-all. But I have seen this movie even more times than I have lived it.