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Use it as a refugium - putting fish there that needs refuge from the hard life of the main tank. Stresses fish are more likely to start disease. Yes you can put an aggressive fish in the refuge if its bothering all the other fish.
I don't really understand how it's setup. I was thinking of doing a tank this way but want it on display so not sure how to really do it, anyone have any advice?
Its a lot of work to set up a refugium. If you really want to do this (there is A LOT of ways to do this), then be prepared to research for dozens of hours. Or we can make this simple with hob overflow and a return that hangs over the rim. You have to learn what is available to you or else you may not be satisfied with the simple setup as described above.
You might be better off with a seperate tank with a simple hob filter and heater