10,000k or 6500k

That is a good idea. It will give more bulb options also. I suggest buy 2 ATI Blue Plus, 1 ATI Aquablue Special and 1 ATI Purple Plus. Placement from front to back Blue Plus, Aquablue Special, Purple Plus and Blue Plus. If you want a darker look switch the Aquablue and Purple bulbs.

 

what size tank do you have, how deep is it?
What is your goal to grow? Most softies (zoas, mushrooms, polyps etc) the light isnt as important. However if you want to grow SPS you definately do not have enough light.

If this is your first tank, I would recommend starting out with zoas, polyps, mushrooms and other beginner corals and if that is the case save your money with the light you have and when you are ready to move up to the more advanced corals then you can trade the light in for something else......either 4-6 bulbs or MH vs LED

 

get a 4 bulb.

 

Blue Actinic Lights are in the lower-mid 400nanometer levels, which 'prime's a coral or plant to begin the photosynthesis process, IE open and turn toward light source and begin enzyme production. They Make the Corals and Fish illuminesce and look great...

But...

Photosynthesis can't occur at this lower light level, not until the 600-750nm level does the coral or plant actually see any real 'food' production.

10,000k and 14,000k bulbs give the 420+ nm needed to activate photosynthesis...

So Blue Actinic bulbs are basically for show, or our own visual appeal. True they can supplement a useful procedure, however, more than enough lower light is produced in the higher K lighting of a 10,000 or 14,000k bulb.

20,000k has too much lumens (brightness) to be of real use unless your tank is significantly deep. Otherwise, it can actually be a detriment as it could 'bleach' (be too intense) for corals near the surface (even halting some SPS coral growth supposedly).



That All Said...

If you want growth and total potential, go 10,000k or 14,000k (unless you have a deep tank ~ 20,000k).

If you want 'beauty' go Blue Actinics.



AKA...

If you put 2 10,000k bulbs in your 2 t5 system,

You have basically the same growing potential as a

4 bulb t5 that is 50/50 (half 10,000k and half Actinic)

Just won't 'look' as good...

Good Article below

Aquarium Lighting; Kelvin, Nanometers, PAR, Bulb, Watt, MH, LED, Light Basics.

 

I don't see why you need a single 4 bulb fixture? You could add another 2 bulb fixture latter, when and if it is needed.

 

good read, thanks.