You'll want a light that's close to the shape of your tank, ideally, so Hydras or anything similar is not ideal. (Go for more of the smaller lights vs fewer larger lights if you do go that route anyway.)
Your tank is long, rectangular. So get a long, rectangular light. Other shapes will make you struggle (and spend) more to cover shadows.
This way the light will best-fit the tank.
Start with that.
Then (based on the lenses used) you calculate the height off the water your light needs to be.
One light I'd recommend is Current USA's Orbit Marine PRO.
- It has 60º lenses.
- They recommend two units for a 24" wide tank
With a little triangle math, you can know that 60º lenses need to be around 9"-10" off the water to generate enough coverage, but not spill light.*
So two Orbit Marine Pro's (model 4112) hung around 9" from the water would do the trick.
* There will always be some light leaving the tank by reflection or spilling unless you put a canopy over the tank.....just FYI. A light that the ideal shape and hung at the right height will minimize this.