My highlight tank is runned on PC lights.
I personally think that PC lighting is much more economical when it comes to lighting smaller plant tanks than MH lighting. Plants do not need nearly the same amount of lights that reefs get. I'd say that 4-5 watts/gallon would be a nice general maximum to go for... I personally won't do anything more than 4 watts/gallon if I can help it. MH lights usually only cover about a two square feet area... a 45 gallon is obviously longer than 2 feet. MH lights are nice for penetrating water, but not for widespread lighting. I suppose if you really wanted to use a MH bulb (I assume a 175watt) you could always place higher light demanding plants near the area where the MH bulb is (I'll assume the center) then place lower light plants near the edges (like java mosses, java ferns, anubias, etc.).
The kelvin rating isn't too critical... its mostly what you like to see. All of my plant tanks run on 6700k bulbs, mainly because I like to see that kind of lighting in my plants. For awhile I did run a 10000k bulb on my experimental 15gallon for about a year, and I got growth that was just as fast as my 6700k runned tanks. Degrading spectrum doesn't seem to do much for plants neither... or that I could observed. I've been running the same PC bulbs on my 70gallon for the past 2 years, and I haven't run into trouble yet... I plan on replacing them when they burn out.
If you are on a budget I would suggest using NO lighting... flourescent fixtures from Homedepot would do very nicely. I believe they also sell flourescent bulbs meant for growing plants. These would work perfectly for a tank and they won't empty your wallet.
Just keep this general rule in mind. More light doesn't always mean more growth. Keeping plants is a delicate balance of nutrients, lighting and CO2 availability. I've seen countless people who run very successful lowlight plant tanks, who later on upgrade their lights in an effort to speed up plant growth without realizing that with more light they would need a higher CO2 concentration in the tank. What happens in the end? Algae bloom after algae bloom. Not very fun. So if given a choice between getting a nicer light fixture, or getting CO2 injection... go the CO2 route first.
HTH
-Richer
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