Going full spectrum!
 

Right now its pretty bland with just neutral/cool white and royal blue. There are however 12 empty slots on my heatsink, so does anybody have any ideas on colors and array? I'm sure I want some UV and this is going over a 20l mixed reef

Here's my set up, UK is unknown

UK RB CW UK RB NW UK RB CW
NW UK RB CW UK RB NW UK RB
RB CW UK RB NW UK RB CW UK
UK RB NW UK RB CW UK RB NW

My answer will depend on how you are driving them, additional 12 all on one string, or multiple strings?

...and how you have the two strings now running all on 100% or whites and blues dimmed to different levels (if so what levels for each channel are they set at?)

If all on one string I'd I'd add 8 violets (either 420nm or 430nm, just pick one or the other) and 4 true blues (480nm) I would also consider replacing the 3 central CW with a high CRI warm white such as the Luxeon Rebel ES 2700K (model. LXW9-PW27. That 9 in the model number is important to get some of the far red that is missing from a white and blue only setup such as yours without going as far as to add actual red leds)

Avoid green, cyans, and reds unless you are going to dim each separately and are prepared to have some disco effect (or use 3up chips which triples the cost)

Zachts nailed it, stay away from the fad colors you have already accomplished that with the neutral whites. Add UV and blues. I have 28 RB, 7 CW, 7NW, 8UV, and 4B on my setup wouldn't change a thing!

sorry i should've specified, the white and blue strings are driven separately by two meanwell drivers from rapid led. I want to get another driver to independently dim my uv and possibly other color's string and was just looking for input on which and how i should array them

Cool, my suggestions stand.