Bulbs

I put in a new 150w bulb. The old ones were 20K and the new ones are 15K, but instead of doing both I just did one. Its a night and day difference. Is this right? Is the 15K better anyways?

 

It depends on what you are looking for. growth, par, bluer , whiter?
It really comes down to personal preference.

 

what about for growth? I don't too much care for the white look, but if its better for the corals then I'll go with whats best for them.

 

Generally lower k rating gets the best growth
Here is a some good info for you
Manhattan Reefs - Sanjay's Lighting Guide

 

Picture not showing for me dee99gt

+1 on above advice anyway, 15k should give you better growth than 20k

just wanted to add, in general there is higher PAR with lower color temp lamps
and bearing this in mind, you may want to reduce your light up times when you swap from the 20k to15K lamp as the difference could cause some corals to bleach until they become accustomed to it

if you have any LPS corals under the 15k lamp watch them for signs of early deflation compared to their behaviour under previous lamps, if they start to close for the day early they are telling you they have had enough of the light

Steve