Quarantine and patience are the two best routes to keeping beautiful healthy fish for a long time. Suggest you search this forum for Quarantine (and Hyposalinity by H2oski if you plan to use) and read and learn all you can. If you do not put a new arrival in Q first, you chance getting into the situation you are in now... in my opinion I would set up a Q system (purchase rubbermaid type storage containers), pull all the fish out and do Hyposalinity, but ONLY IF YOU HAVE OR PURCHASE A REFRACTOMETER to accurately measure the salinity/SG. Best buy on the net is here $70:
<a href="http://www.***************.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=PA&Product_Code=REFRACT-PA&Category_Code=Hydrometer" target="_blank">http://www.***************.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=PA&Product_Code=REFRACT-PA&Category_Code=Hydrometer</a>
... Now if you don't have the time/patience/$$$/inclination to do hypo (which is the least stressful to the fish) then in the Q system use copper with Maricyn2 (I have before with success).
Now, about the bio-filter (which by the way is a MUST if you don't want to loose fish to ammonia/nitrite spikes/levels)... if you want to use some of your already cycled media (bio-balls) from your display set up... just take any bucket and PVC fittings, drill the bucket put fittings in bottom, put the bucket above your container level so you can pipe the water back to the container (gravity feed) and use a pump to get the water from the Q container up to your home-made drip filter. If you want take a plastic drip pan made for plant pots and drill holes in it to make a drip tray in the bucket. I have done this with a 5 gallon Coralife salt bucket (lots of bio-balls!) and have kept many fish simultaneously in Q for months with no problems. It sounds like a lot to do but it is the RIGHT WAY to keep and stock saltwater fish. There is a great product that eases the fears during Q. If you in any way think you may not have a cycled bio-filter to use during Q, I highly recommend using some "bacteria in a bottle". I have used the Tropical Science brand many times with seeming success. You can get it here:
<a href="http://www.**************.com/product/prod_Display.cfm?siteid=6&pCatId=4003" target="_blank">http://www.**************.com/product/prod_Display.cfm?siteid=6&pCatId=4003</a>
for what I consider a reasonable price considering the cost of your fish (investment).
Just my .02, and best of luck, Joe.