1. Tangs are not good choices for newly set up aquaria. To thrive they need a stable environment with pristine water conditions. Anyone thinking their newly set up aquarium is stable is invited to read through the following thread:
Maturity Issues
2. Tangs are medium to large, active fish that need plenty of space. On the question of tank sizes, we defer to Scott W. Michael, who recommends the following minimum sizes in Marine Fishes, 500+ Essential-To-Know Aquarium Species, Microcosm Ltd., 1999:
1. Acanthurus achilles, Achilles Surgeonfish, 100 gal (380l).
2. Acanthurus coeruleus, Atlantic Blue Tang, 75 gal (285l).
3. Acanthurus japonicus, Powder Brown Tang, 75 gal (285l).
4. Acanthurus leucosternon, Powder Blue Tang, 100 gal (380l).
5. Acanthurus lineatus, Lined or Clown Surgeonfish, 180 gal (680l).
6. Acanthurus nigricans, Whitecheek Surgeonfish, 75 gal (285l).
7. Acanthurus olivaceus, Orangeshoulder Surgeonfish, 135 gal (510l).
8. Acanthurus pyroferus, Mimic Surgeonfish, 75 gal (285l).
9. Acanthurus sohal, Sohal Surgeonfish, 180 gal (680l).
10. Acanthurus tristegus, Convict Surgeonfish, 75 gal (285l).
11. Acanthurus tristis, Indian Mimic Surgeonfish, 75 gal (285l).
12. Ctenochaetus hawaiiensis, Chevron Tang, 100 gal (380l).
13. Ctenochaetus strigosus, Kole's Tang, 75 gal (285l).
14. Naso brevirostris, Spotted Unicornfish, 180 gal (685l).
15. Naso lituratus, Orangespine Unicornfish or Naso Tang, 135 gal (510l).
16. Naso unicornis, Bluespine Unicornfish, 200 gal (760l).
17. Paracanthurus hepatus, Pacific Blue Tang, 100 gal (380l).
18. Zebrasoma desjardinii, Indian Ocean or Red Sea Sailfin Tang, 135 gal (510l).
19. Zebrasoma flavescens, Yellow Tang, 75 gal (285l).
20. Zebrasoma scopas, Brown Tang, 75 gal (285l).
21. Zebrasoma veliferum, Sailfin Tang, 135 gal (510l).
22. Zebrasoma xanthurum, Purple Tang, 100 gal (380l).
You will note that none of these minimum sizes are less than 75 gallons. If you have a tang in a smaller tank, you should expect that some people will give you a hard time about it.