How big is your tank?What kind of sailfin is it? A purple sailfin tends to be more agressive as a yellow one, i think. Do they eat well? Do you have enough hidingplaces?For how long is this tank running? Do you keep more fish in your tank, ifso which?
I have so many questions....Anyway, you could try one of the following options.
1: Put the copperband in your sump for a week. This gives you the chance to get it well-fed and it gives your sailfin a while to get used to the copperband (they smell eachother).But remember it's always a risk, copperbands are very delicate fish! If the fish is already in bad shape i would try option 2.
2
ut the sailfin in your sump for a week.(poor guy...
)
Try to replace some live-rock to create new territory before adding the sailfin to your tank. This will also be stressfull to your sailfin but if it is in good condition, and you keep an eye on him,it'll do fine.
3: Rebuild your decoration.
Try to create many hidingplaces and caves and make a few towers of rock to make sure they don't have to see eachother all the time.
This will be very stressfull too for your fishies though...:doubt:
and not so good for the bacteriaculture in your system.
I hope it will work out with your copperband..
Good Luck and keep us updated!