Tangs in a 90?

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I have a 4" Yellow Tang and a 3" Hippo tang in a 120 gallon tank.

They are healthy. I have noted that the Hippo Tang can swim from one end of the tank to the other in about 0.1 seconds. When they get bigger in a year or two they will probably be crowded.

On the other hand I believe these fishes' survival in my tank will be better than it would be in the wild.

In the wild these fish will be subject to predation. In addition, they will have to compete with other tangs for territory in order to feed. The weaker Tangs in the wild will die because they are not strong enough to compete for territory or food. That is why, with a population of Tangs that is not increasing or decreasing, out of hundreds or thousands of babies (larvae, fry or whatever) spawned, only two will survive to breeding age. This just simple math, if more than two fish survived to adulthood from each pairing, the population would increase.

In other words, the population of Tangs in the wild will expand until it is limited by predation or lack of food supply, and the overwhelming majority of Tangs will die before they reach breeding age.

My tank is not a perfect home for two tangs but it is probably a more comfortable and healthy environment for them than the cruel ocean.