There certainly can be many conflicting opinions whether or not you can keep certain marine fish (most notably tangs) in certain sized aquariums. This can even become an emotional topic for many hobbyists.  However, as the plural of anecdotal is not “fact”, I would challenge some of the commonly accepted facts / opinions that you read about on-line and further suggest to you to research these topics for yourself.  In my opinion, completing your own research from credible sources is the only way to demonstrate responsible fish keeping though focusing on the facts.  The below is a few examples of how I will approach keeping tangs to help give you better understanding of what I am talking about. 

1) Proper Sized Aquarium

Don’t be fooled, conventional rules of thumb on tank size should not be considered a hard fast rules, but only used as a starting point to determine proper tanks size. That is why most people state tank sizes as “minimum aquarium size” and not as “ideal aquarium size”. I have seen tangs which appeared to do well in a 120 gallon tank for many years, and yet I have seen the same type of tang suffer to the point of eventually dyeing in a 230 gallon tank. So which tank is was the correctly sized one?  And just to make matters a little more complex, different tangs have different requirements so you do have to keep in mind, not all tangs will have the same needs. How long and wide a tank is along combined with how much swimming space they would need is what you will need to determine in order to find a properly sized tank for the specific tang you would like to keep. I have always found the amount of open swimming space was one of the most critical factors to think about. Having a tang in properly sized tank will do no good if you use up the space in the tank by placing large amounts of rock and/or large amounts of tall corals.  In the example I just mentioned above, the 230 gallon aquarium had far too much rock in it (at least in my opinion) and had a lot of extremely large SPS corals which limited the amount of swimming space to less than what was in the 120 gallon tank. That is why I choose to stock based on the open swimming space (open water area) within the tank after I have all my planned rock and corals in there. I will typically refer to published research papers and books written by credible authors in the field when determining the proper aquarium size and swimming space required.

2) So how big will my tang get?

Many sites will list the maximum scientifically measured size of a fish as their adult size.  I think it is more realistic to look at the average size of a fish.  After all, the tallest person in the world was well over 9 feet tall, yet all of the doorways into our homes we build today are around 7 feet with around 8 to 9 foot tall ceilings inside the home.  This doesn’t mean you or I were stunted by our home, just that we are closer to average height as compared to the tallest person in the world. There are a few other factors that also make the average size of a fish size as a better approach to use

–          In the wild the more dominant male, or “super male” will tend to get larger than normal which will not occur within a home aquarium

–          Fish will not get as large in captivity as they do in the wild.  I have read studies that showed fish in captivity typically get to between 66 and 80% of the normal or average size in the wild.  These same studies also concluded these same fish were not stunted due to their: healthy appearance, normal behaviors, and long life span.

Once again, I will typically refer to published research papers and books written by credible authors in the field when determining how big a fish will become over time.

3) What about keeping multiple tangs?

For long term success, you should either keep a single tang, or a group of around 5 or more assuming your tank size and filtration can accommodate them.  Many different types of tangs will typically not tolerate other tangs of the same species or a species that look very similar to themselves, while others can be just plain bullies.  You will always have at least a small element of risk with keeping multiple tangs as these fish can be a lot like people, each with their own personality and disposition. Sometimes you can complete more than enough research but you just won’t know for sure how it will work out until you try it.  Just be prepared to watch carefully and react quickly should things not work out the way you expected. Once appropriate tank size, fish size, and swimming space has been validated, you will need to determine what type of territorial needs each fish has if you are keeping more than one, and what would be the best approach to take when adding these fish to your set-up.  I would include all fish (and not just tangs) into this part of the process.

Many have suggested keeping tangs in odd numbers, like 5 or 7.  I have often wondered about how important this actually is, after all, can tangs count?  I would guess the most critical element would be to keep them in higher numbers, the more the better (assuming you are not overstocking the tank as already mentioned).  However, I will typically refer to published research papers and books written by credible authors in the field when considering keeping multiple tangs in a single aquarium.

4)Are there other things to consider ?

Putting all personal opinions aside, we can all agree that good aquarium husbandry will be just about the most important aspect of keep in any fish.  The two other critical elements that will also have to be considered are:

1)      Water parameters
2)      Diet

This can be just about the easiest part of your research to complete.  As the tangs that I keep come from the in or near our coral reef, I try to keep the water parameters as close as that of the coral reefs as I can.  The below link can explain that in a little more detail.  The proper diet is also fairly easy to identify with a little research from published research papers and books written by credible authors in the field.

https://www.reefaquarium.com/2013/the-basics-of-marine-aquarium-water-parameters/

5) How do I know when I have made the correct choices?

When you have completed the proper research, things should be going great and you will be enjoying your aquarium for years to come. So, what does it look like when things are going good? Basically, you should have a healthy fish that looks and acts normal.  The below list are just a few things that will give you a indication when things are going well:

-No signs of chronic disease.
-No signs of abnormal coloring or body shape.
-No signs of abnormal behavior.
-Strong appetite when offered a proper diet.
-No signs of abnormal stress, which is typically shown as higher than normal respiration rates.

There can be many different signs to watch for which can be a indication that you may not be providing a proper home for your tang, or any other type of fish as well.  You can take the list above and watch for the opposite.  However, there are a few other things to watch for that could be a indication your fish is in a aquarium that is too small.  These things can include:

-Bruised nose.
-Bent or torn fins.
-Shortened life span.

I’m not trying to make things seem more complicated for you when trying to determine proper aquarium size and environment for your planned fish.  I just want to give you some guidelines to think about when completing this research for yourself based on what I have learned completing my own research along the way.  There is a lot of information available on line and in forums, but not all of it is good.  You need to be able to find credible information from experienced people and start form there.  The better quality information that you find, the easier the research will become and you will have a better and healthier aquarium because of it. 

Some additional references:

http://coralmagazine-us.com/content/arrested-tang-police-i-refuse-confess

http://microcosmaqx.typepad.com/jay_hemdal/2009/01/beware-the-tang-police.html

http://www.tfhmagazine.com/saltwater-reef/feature-articles/putting-fish-back-into-the-reef-tank.htm