Keyword: years ago.
I'm running Hydra 26 HD's
again. I ran 3 on my 36" SPS dominant tank with amazing success on my Acropora. Only went to my ATI 6x39 because my Internet was terrible in my house. Adding lots and lots of acropora, I really enjoyed using the Acclimation Mode the light and many others come with. Comes really handy!
That being said, I downgraded recently since I'm moving and went back to Hydra 26HD over my Reefer 170. What're my noticeable results so far from an ATI T5 to these LED's? Almost noting. I'm getting just as great of growth and color as I did with my T5.
The only main noticeable effect I have was the brightness of the tank. While my Hydra 26HD is producing the same results, it makes the tank look dimmer ONLY BECAUSE it's 2 pucks over a 24x20 tank and I'm running a mock Ecotech Marine SPS AB+ which makes it look quite dim. Where as obviously T5, you'll get that fully and bright look.
Now for coverage, this is all depending on what corals you're wanting. If you're planning more SPS, without a doubt I'd do 3 MINIMUM. If you're doing mixed softies and LPS, 2 would be fine. Though I feel you and your corals would be happiest with 3. Though 2 for the time being would be plenty sufficient!
These pictures were from my 36x18x19 tank where I ran two Hydra 26HD's and eventually adding a third.
Here's how it was before and after shots.
On the Teal acro, that was under T5's. Notice under Hydra 26 HD's it began developing a wild look pigment. Never got that before with T5.