Yup. Exactly. You are isolating the system (since there's no movement of water "backwards" to the main system), and creating stability through a potentially pretty huge daily water change (relative to the volume in the smaller system). You may have to do top-off into your frag tank, depending upon how large it is - you have to be doing some hefty water change volume (40-50% in my case) before you can avoid ATO altogetherPerfect- and you have it set so that the extra water coming in from the main tank AWC just displaces old water over the overflow into the drain?
I’m going to try this on my frag tank. I was looking at a long return and supply loop since it is 40 feet away from the sump, and there are several doors etc in the way.
This way- I can have a 3/8 line put AWC water into the tank, and put the excess down the drain 4 feet away.
It's fascinating being able to create a different "mindset" for that tank though. Want to dose tons of phyto/other smaller food? Go nuts - massive water change will save you from worrying about nutrient buildup, and since the flow through the tank is super slow whatever you put in there will stay there for ages. Want to keep that non photosynthetic coral/filter feeder that doesn't do well unless it's got tons of food? It's now a biological filter in a heavy nutrient environment.