Looking for some good lights to really grow coral. What is the best one and why? I am eventually going to get a large tank but now have a 5 gallon.
This question is difficult to answer simply, because the question is very broad in it's context.
For instance, here is some of what I know goes into the definition of 'best light to grow coral' -
-What type(s) of coral will be kept?
This matters greatly, as the requirements can vary by an order of magnitude (ie- 10x).
Gorgonians can have requirements which are very low, SPS very high, some varieties are non-photosynthetic altogether.
-How far from the coral will the light(s) be mounted?
Light irradiance varies as a function of the inverse square of the distance (see- inverse square law:
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/isql.html)
Simply put, if the irradiance at 1 unit of distance is x, at 2x=1/4x, and at 3x=1/9x, ...and so forth: 4x=1/16, 5x=1/25...
So to sum up, relatively small changes in distance can make a large difference in how much light strikes a coral.
Because of the above, it is beneficial for a light to be adjustable, so you can compensate for the effect of both distance/irradiance and type(s)/needs of corals being kept.
Look for lights which can deliver between 100-300 PAR at the distance where your corals will be placed in the aquarium from the light (see:
https://en.wikipedia.org/wiki/Photosynthetically_active_radiation)
Look for a light which will deliver a relatively uniform distribution over the entire area in the aquarium to be illuminated.
Look for a light which is capable of producing/tayloring the spectrum for the effect(s) and visual impact which you desire.
Some corals respond differently to certain spectra, such as in coloration/growth/tolerance/etc., regardless of PAR values.
Some spectra may be more pleasing to your eye, for example.
Those are some of the variables, and no need to reinvent the wheel here.
Look to other's experience for direction, unless you are a scientist at heart and enjoy investigating such things.
Here is a thread from nano-reef.com, which is a web site specifically dedicated to small tank environments for your search. Hope this helps
https://www.nano-reef.com/forums/topic/386776-5-gallon-reef-light-again/