your concern isn't at the watts the LED puts out, it would be more of the PAR it gives. From the site, it doesn't really tell me much as it lack of information.
few things missing from the site that is important which you might be able to tell us.
Do you know the PAR reading by any chance?
How many LED diodes are there in total, or what would be the watt per LED?
The answer of these two questions would pretty much tells you if it is enough for your tank.
If you donno about the PAR, then 2nd question is very important. If you have the fixture that made with all <3W LEDs, which means the true output is no where near 40W. The smaller the LEDs it uses, they lower the actual Watt it can provide due to the smaller watts LED usually would use much less current.
For example a 0.5W LED might only be able to sustain about 40% of the max current to give it the proper life span, which makes the 0.5W really only a 0.2W.
There is also couple other reason your coral not reacting well, maybe the light wasn't providing the proper spectrum. Or simply just your corals don't like LED lighting if you had T5 or MH before.
For a 24" tank which is not really a long tank and 20" tall is just the right height for most LED fixtures, I think "40W" is enough as long as it gives out enough PAR.
I had a 72W on my 24x24x24 tank before and everything working well even SPS.
