Actually, it would hold less because you are using outside measurements. If you used inner dimensions where the water actually exists, it is about 19.8g for the larger one, and 17g for the smaller one.
These are quite small actually, but if your tank doesn't drain much when the return pump is off, or the system is used as a refugium only, it isn't bad.
It is DIY, as you are going to have to assemble it yourself. And what you see is not what you get. There are several items in the pictures that are not included.
That is the exact vendor & product someone showed me a few weeks ago.