You orig. wanted just a supplement to your Kessils, right?
Your depth is fairly minor all things considered at 14"..
first thing if you keep the Kessils is to optimize the placement.
This vid will start at the appropriate point re: height and linear spacing for 2 90W kessils.
Keep in mind they are optimizing for a 24" cube per each light.
At your 18" wide you can adjust slightly. I/e go 6-7" not 8" height.
Adding something like this is probably sufficient.
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Adds 216W to your 180-ish.
Enough for a shallow 50gal..

Of course you will need to suspend it afaict.
Greets all. Apologies if this is something that's been beaten to death. I thought I'd post how I mounted my Kessils to a 61" hybrid fixture, and what screws I used. I ordered the 61" Hybrid Fixture, and 4 Kessils. The I used the rectangular brackets. There are enough in the fixture kit to mount...
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