I have a 220 with dimensions of 72L x 30H x 24W. I currently have three gen 3 pros on it right now mounted on a rms slide mount. I want to upgrade to the gen 4 pros. For livestock I have a mixed reef sps lps and some softies. I received one gen 4 pro for Christmas and I have a xr15 gen 4 pro I am currently not using. Do you think I can get by with buying an additional xr15 gen 4 pro and place the two xr15 on the ends with the xr30 in the middle. I also plan on getting the diffusers as well. Thanks for the responses.
That depends on what you are going to keep under them. If you kept the higher-light stuff under the 30 and the lower-light stuff under the 15s, then maybe. A 15 over a 2x2 area at 30" deep is not a lot of light.
This is a really tricky question because it is all about the inhabitants and placement.
The diffusers will cut the output - I would skip them if you just use the 15s even if they will blend the colors better.
I also saw someone who has two XR30w pros on the ends and a XR15W in the middle. The problem with my tank is there are support braces on a 220 which breaks up the open space into thirds .