20A minus a 20-30% cushion leaves you with ~17A. If you run 700mA buckpucks (which probably pull 750mA or so) you can power 22 of them. 22 buckpucks at 24v is 132 LEDs.
If you use the right LEDs, 132 is probably overkill for a 120g tank.
Keep in mind though that the power supply is pretty much the cheapest part of the build. Most people designing this system from scratch would go to mpja.com or a similar vendor and get three or four of their 6.2 or 8.3A supplies, which are around $15 - $20 each. So this behemoth is saving you $60 plus or minus (ironically it would probably cost more than that to buy).
The one disadvantage is that you can't use a regular plug-in timer to turn "chunks" of your array on and off at different times if you're using a single power supply. You'd have to use something (relays driven by timers or a microcontroller) that could switch between the DC supply and your buckpucks, or use the dimming circuit on the buckpucks to turn each string on and off.