Relative merits of PC, T5, T5HO and MH lighting in various tanks


Lifetime Charter Member
Lifetime Member
May 4, 2006
I no longer understand the different lights, how to approach mating a particular technology (PC, T5, T5HO, or MH) to different tank sizes and approaches. By sizes I mean width, breadth and depth, realizing that the spread of light from its housing, shape and efficiency of reflectors and how well it can penetrate(?) the water to depth. (Actually, I'm not sure if the way I phrased that last bit is even scientifically accurate, as why would 150W of MH light penetrate more than 150W of PC or T5/HO light?)

I do realize that the choice depends on important variables such as what one wants to accomplish: a low maintenance ("low light") tank; a high-growth, any-plant-goes, CO2-enriched tank ("high light"); or somewhere in between. I realize that tanks below some threshold in size (10 gal?) cannot use the watt-per-gallon rule of thumb as an accurate guide.

To this end I have gotten myself a PAR meter with the idea that PAR is a more "absolute" measure than watts or light type. However, I do not have access to all different light technologies, bulbs with different kelvin temperatures, reflectors, etc.

What I really would like to know, and perhaps this might be a good article subject for the Report, is how are the PC, T5, T5HO and MH technologies different from each other. What are their relative pros and cons with respect to PAR light output, heat generation, cost ("dollars per watt" or "dollars per X µmol m-2 s-1"?), longevity (how often do bulbs need to be replaced?), and that highly subjective aspect of their light quality aesthetic.

I know this is a BIG question, but I know I can't be the only one a bit lost. I think having more information about the various technologies as they apply to planted tanks would be helpful.


Tom Barr

Staff member
Jan 23, 2005

That's what you want here.

Do not get confused, there are plenty of folks out there after that specific goal, sometimes unbeknownst to themselvces unfortunately, belieing all sorts of kooky things and theories, methods that are needed if only someone would do the research, or using data that does not support the claims.

Say I have a 180 Gallon tank.
I have enough $ where the $$ is less of a concern as I'd rather have something look nice and be flexible for most any light need I might have.

So, I like HQI, they are bright, but perhaps too bright if something is out of balance or if I want to slow things down, but not stunt growth with super short photoperoids.

So if I add T5+HQI, now I have a nice mix, I can drive things faster, or I can drive things slower.

Or I can use the T5's for 10 hours and the MH's for 2-4 for a mixture.

If I leave for 2 weeks, I have the ability to use only the T5's.
If I am around often, and want to garden more, I'll drive the HQI for more, say 6-8 hours.

This way I can do what I want.

I have a lot more practical experience than most do about what types of lights are good on a specific tank. I also have far more testing experience using lab PAR meters than anyone in the hobby.
Not tooting the horn here, however, I know enough about light to know that all these folks claiming the W/gal rule is no longer useful are full of beans.
You can measure the nano tanks and they are less in smaller tanks in terms of PAR vs the watt/gal. Cheapy bulbs vs the nicer electronic ballast using CF's might be one reason. Angle of strike is another, the light does not spread out much. I just think many of the smaller CF bulbs suck.

You can use a lot more w/gal ona smaller tank also from a practical standpoint.
10w/gal is not hard, whereas on a 100 gallon tank, it becomes very costly.

Also, CO2 mixes readily in small tanks, not so in larger tanks with much less flow/mixing.

Tom Barr