Jason... very good to hear, my friend! And thank you for understanding and putting up with us old salty dogs
Stocking densities for aquaria are largely subjective... for example, an "inches of fish, per gallon of water" rule of thumb would not fairly measure the burden of ten 1" long cardinalfish versus one 10" lionfish in a thirty gallon aquarium. Both are 10" of fish length... but two very different bio-loads as you can imagine! The same holds true for active swimming fishes (even surge species like several of the tangs) which need remarkably long and high flow aquaria versus very sedentariate fishes like anglerfish. Here... a 10" anglerfish in a 4' long 120 gallon species tank could work.... yet a 10" adult Sailfin tang would be a crime.
In these gray areas hide the keys to success with stocking densities by researching a fishes history and natural behavior and biotopes. Research as much as you can about a species before buying it. I checked the fishbase.org link above and it is working for me (perhaps just a glitch that night/when you checked).
Also... do check out the thousands of pages of free content (articles, pics and FAQs) that we have archived at
www.wetwebmedia.com
These are but two of the many accurate and objective (nothing to sell you) resources like this message board to tap in your searc for data.
Regarding your present state with the tangs in possession... there is some room to move regarding compromises you make with husbandry and hardware (water changes, filtration, flow, etc)... but, FWIW:
I would lose the sailfin tang for its large adult size and likely (severe) aggression in close confines as it matures. Then... I would keep only one of the remaining Zebrasoma species (purple, yellow, scopas) as mixing within the same genus is tempting fate/aggression down the road again. The scopas gets my vote for slow growth, usually gentle nature... and absolutel under-rated beauty as an adult (jet black). For a second tang... keep one of the remaining non-Zebrasoma species... with Achilles and the Powder Browns being more difficult to keep than the hepatus Blue. Too bad the Blue gets so large. A choice to make here.
For remaining tank mates... there are many wonderful choices of colorful hardy fishes that stay medium sized (say under 6" as adults). Metallic blue hamlets, multicolored wrasses, fantastic dwarf angel species, etc.
Best regards,
Anthony