FYI her friend is Lee aka Bulletsworld and she's been on this board for a long time and knows what she's doing. Lee has set up a 230g tank and she used mature LR but for some reason it's taking awhile to cycle. She bought the unicorn tang from someone on this board who was shutting down his tank, and that is why she bought it before her tank was fully ready. The tang was eating in the person's tank who she bought it from. Since the move to Diana's 90g holding tank, the tang ate at first but now is not eating. Possibly due to stress from being in a small tank (which was why Diana was considering moving it into her display) but it's hard to say for sure.

As large a tank as she is planning is insufficient for a unicorn tang.
The unicorn tank grows to 27 inches and therefor requires a least a 3 foot wide tank and a length of 9-10 feet.
They really do not huge tanks.

The tang should NOT be put into your display (at any cost).

If you do, here's what you risk:

1) All out war between the new tang and the older tangs.
2) Stress that will cause ick or other problems to breakout.
3) Possible injury or loss of your prized Achilles.
4) Even greater stress & huge hassle when trying to catch the fish out in another couple of weeks once Lee's tank is finished cycling.

Your display tank is "perfect" right now, but you will risk all your hard work and patience in doing things just right, simply in the hopes of temporarily housing and feeding someone else's fish. Consider how you and she will feel if you lose one or more of your fish by adding in this new one?

Adding another small tang into the 90g is by far the safer solution, since the risk is isolated to the new tang & not all the fish in your display.

IMO, you have to make this decision with your MIND and not your soft HEART. I know how much you love your fish, so why risk having anything happen to them?

This may sound cold hearted, but I've kept stuff for other reefers in my display that I later regretted due to the resulting problems. You have to do a risk-reward assessment and decide whether the reward of possibly having the fish eat in your display is worth all the risk mentioned above and by other posters.

Anthony

Just wanted to say your tank is my dream tank. I saw it long ago when I bought this house with the 44g in it. I believe your tank is an exception to many rules with all the free swimming space... I'm going to look at your webcam now... and drool like I always have.

Thanks for all the advise. The Unicorn is in the 90g still, and I spend more time interacting with her then before and she seems to be doing better. Lee brought the Unicorn's old tank mate here to keep her company, but although she shows a feeding response, she is still not eating. She is very fat and healthy looking so I am not overly concerned just yet.

I do not want to risk my other fish and this is why I asked if it might or might not be an option. Thank you everyone for helping me decide.