So a 10K will have greater PAR than a 15K, which in turn has greater PAR than a 20K. And lower PAR generally equates to slower coral growth.

 

There are some exceptions, but generally the lower K / higher PAR bulbs are better for growth, the higher K bulbs for colorization and  "pop".

 

But PAR is only 1 factor in determining coral growth. Things like total photperiod,  water temperature and temperature stability / range; levels of Alk /Ca / Mg and even Strontium and Boron - and again, stablity of those levels; water flow in the tank; availability of nutrients / food in the water column; even fishes and are they "picking on" the coral at all... All those things help determine coral growth; not just PAR levels.

 

Personally I think people give too much weight to PAR alone. The best reef tanks I have ever seen usually had decent lighting - but not necessarily the most intense / highest PAR lighting. But they almost always excelled in most or all of the other areas I mentioned...

 

Also, there are ways to help make up for PAR loss when going from a 10K to a 15 or 20K bulb:

 

 - run your lights for a longer photperiod (maybe an extra hour or 2)

 

 - lower your lights closer to the water

 

 - raise your corals higher in the tank 

 

 - add supplemental lighting to boost the overall output

 

 

IMO, I would choose the Kelvin color of lighting that appeals to me visually first; not necessarily which K rating puts out the highest PAR.

 

 

 

 

 

 

I also noticed less growth from my 20k bulbs which was kind of demoralizing.  I have since decided to purchase the phoenix 14k bulbs since I hear such good things about them and keep the 20k as a backup.  I think its all in the combination of t5/pc and halides.  Whenever I visit someone with a nice tank I always ask about the lighting.