Another aspect of a skimmer that shouldn't be overlooked is the O2-Co2 relationship which can be seen through Ph testing.
+1
Definitely good to be aware of, but maybe just as important to know that CO2 buildup* is a function of over-stocking.
A "well stocked tank" is one that is stocked within its limits. One of those limits is gas exchange, which is largely dictated by surface area of the tank. (This is one of those points you're liable to find in a
book from the olden days when reading was popular, so that makes it especially good to bring up.)
Of course air-stones and skimmers help (little more than a fancy air-stone containment unit, after all

), but are not the only thing that can help.
Most important of all, this is a limit that the owner has to take time to see/find out and a limit which most folks scoot right past without noticing on the way to overstocking their tank.

That is not the only way to do it. (Not even a very good way. But it is the most popular.)
* unrelated to O2 depletion, which is also a potential concern; unrelated to household CO2 buildup, which is a function of the house