This is lengthy - sorry.
I used the 10k in the 250w and the 12k in the 400w. I have a 10k 400w but have not used it yet. The 12k is closer to 14k than 12k although you can tell a difference. I have 12k 400w in the center and 10k 250w on each side.
*************One note of caution*************
My M59 probe start ballast would not fire the 400w bulb, nor would a friends, nor would it with a new capacitor and the ballast/cap themselves were less than 3 months old.
It was necessary for me to purchase a pulse start ballast to fire this bulb. Reeflux recommended this also, although if you look at aquarium-lighting-guide.com, they will not say that a pulse start is required and I do not think you will find it on Coral Vue's site either.
Another thing.............if you look at Hello lights site they say...........
* Pulse Start ballast. We recommend you use this on 400W Ushio 10,000K and Radium 20,000K lamps. Most other 400W bulbs should be fired on an M59 ballast located here.
FAQ:
Q. Can I run a probe start style MH lamp (bulb) on a pulse start MH ballast and vice-versa?
A. The manufacturers of the ballasts and lamps do not recommend this. What adverse effects will it have? Ballasts/lamps are designed to be run as a circuit within certain tolerances (line input, starting voltage, current, etc.). Electrical components should be operated as specified. By not following these guidelines, you may increase the chance of risk, damage to your equipment, premature failure of lamps and other potential hazards. Lamp manufacturers do not guarantee lamp performance or reliability if incorrect ballast is used with a lamp.
SO in my opinion - the Reeflux Coralvue bulb is sort of a bassstard bulb. The way I understand it - the bulb either has an ignitor or the ballast has one in the ciruit. The Reeflux setup has an igniter in the ballast setup and some sort in the bulb also.
It requires (at least sometimes) a ballast to fire it that it should not. And another thing Coralvue is not quick to tell you that this ballast is required, just recommended........ and finally they sure are not quick to say - "hey send it back".
Buyer beware - if you do not have an Electronic ballast, it may be good to stay away from Reeflux Coralvue.
I did use my old M59 ballast on the 250w bulbs so far with no issue, at least 4 bulbs. I now have a 400w RF bulb that I am using a probe start ballast with. As you can see from what Hello lights says, I should not be nor should I use it with other M59 style bulbs.
Someone said (Premium Aquatics) that perhaps if you fired it up once with an Electronic Ballast, it would then fire with the regular magnetic M59 style - I tried this, it still would not fire. It may work on some setup, but it does not always work.
As for coral, I was using a generic MH bulb prior to Coral Vue Reeflux and could not believe the color up in the SPS after just a couple months = big difference - I like the bulbs, but have only been using MHs for about 2 years so not so much experience with them vs other brands.