People will do what they will in regards to tangs. I have a 1" Yellow in my 29 gallon right now hoping that he will take to eating the darn Caulerpa that has invaded my tank. If he does, he will be gone when complete, If he does not, he will be gone within two months. That being said here is some information on keeping tangs for what it is worth.
1. Tangs are not good choices for newly set up aquaria. To thrive they need a stable environment with pristine water conditions.
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.
3. The above sizes assume a standard rectangular shape with plenty of open swimming space. If your tank is full to the brim with coral, your tang won't be happy. Tanks 125G or larger are assumed to be at least six feet long (approx. two meters). Where possible, choose longer rather than deeper shapes. While smaller species may be OK in a four foot tank, they will probably do better in a five foot or longer tank.
4. We do not condone the practice of keeping more than one specimen of the same or very similar species together in the same aquarium, except possibly for Zebrasoma species in a very large tank.
5. If you wish to keep multiple dissimilar tangs in the same aquarium, we recommend you have an aquarium significantly larger than the sizes listed above.
