5 gallons? So you're gonna want to use it as a quarantine tank? Is it big enough to use as a QT?

Separate you will new another system almost complete.

If you tie it to your existing system proximity is key. Small pump to feed. A return overflow... And you're done. Just add lights.

You could also mod a section in your sump for grow out.

Remember that fragging can be harsh on the coral. Putting them immediately into another system adds to the stress. But it the two are linked...