Hm. 15" is still pretty narrow to try and service with a single light....
"Ideal" mounting height for a fixture with 90º lenses would be about 7.5" off the tank.
This avoids dumping large quantities of light outside the tank, but effectively only lights about half to two-thirds of the tank.
You would be much better off with multiple smaller Hydra's.
In terms of switching, only the Orbit Pro can really get the 24" depth you have, and I'd suggest two of them.
Only one if you are going to add on to the Hydra vs switching.
Having said all that, let me take a step back.....
Do you use a light meter so you actually know how much light you're putting in your tank now?
If not, stop what you're doing and get a light meter right now.
At minimum, a lux meter like the "LX-1010B" I use is only about $15, delivered, and works quite well.
If you can swing a PAR meter, so much the better. But don't go without!
After you order something (and while you wait on shipping), you can also download a free lux meter app to your smartphone to play with.
Not all apps are 100% compatible with every smartphone camera, so do a little experimentation to make sure you're getting good readings before you start assuming anything. (i.e. measure a known quantity like sunlight)
Sorry this post reads like a giant list. ;)