I would not continue to add multiple tangs. Tangs will stress easily and multiple tangs are never a good idea IMO unless in a very large tank (i.e. 500++ gallons). When I added my Achilles to my 180g it had one or two spots of ich on it and with a few days it was gone and never had any ich on since in over 2 years. I would never add another tang in my tank. Not enough room and a 180g is probably too small for an Achilles.
Naso's, Achilles', Power Blue's and even yellow tangs need a lot of swimming space especially length wise. Height of the tank does not matter as much. With too many tangs and not enough room there will be stress issues and then ich breakouts.
Here is my advice: Do not add any more fish to your tank. Make sure your ammonia is zero for a couple of months. Make sure your nitrites are zero for a couple of months. Make sure your nitrates are under 20 for a couple of months.
Once your parameters are under control for a couple of months then consider adding your next fish of small to medium size. Use quarantine as others suggested. Do not add any more tangs ever if you still have any in your tank. No more additional tangs period (no Naso's, no Unicorn tangs, no achilles or powder blues, no more tangs)!
Once you have added your next small to medium fish and it has been kept alive for a number of months consider your next fish. Plan out the fish you want. Do not be impatient or spontaneous and buy large or aggressive fish that will stress out your other fish. At some point you will have to decide you have enough fish and be satisfied with what you have.
If you plan correctly and take your time all of your fish will live very long lives.
Naso's, Achilles', Power Blue's and even yellow tangs need a lot of swimming space especially length wise. Height of the tank does not matter as much. With too many tangs and not enough room there will be stress issues and then ich breakouts.
Here is my advice: Do not add any more fish to your tank. Make sure your ammonia is zero for a couple of months. Make sure your nitrites are zero for a couple of months. Make sure your nitrates are under 20 for a couple of months.
Once your parameters are under control for a couple of months then consider adding your next fish of small to medium size. Use quarantine as others suggested. Do not add any more tangs ever if you still have any in your tank. No more additional tangs period (no Naso's, no Unicorn tangs, no achilles or powder blues, no more tangs)!
Once you have added your next small to medium fish and it has been kept alive for a number of months consider your next fish. Plan out the fish you want. Do not be impatient or spontaneous and buy large or aggressive fish that will stress out your other fish. At some point you will have to decide you have enough fish and be satisfied with what you have.
If you plan correctly and take your time all of your fish will live very long lives.

