Originally Posted by pelon_mx
I'm rebuilding a 70 gallon fish tank I bought years ago and since then it still has the NEC Bio Lux (6500K) installed and running, after reading all the posts of Byron and a lot of people more it says me that installing a new lamp is a must, As this tank will be a planted I'm also convinced I will have to install a second lamp to get at least a little bit mora than 1 Watt a gallon. The guy from the LFS recomended me to replace the lamp for a new 40 watts but with a temperature of 18000K, is this idea a good one and can I start a planted tank with this replacement only or do I still have to add a second lamp?
All sugestions are welcome
The kelvin number (18000K) is the colour temperature of the light; this comes from the light spectrum, the breakdown of the colours that combine to make up light as we see it. Mid-day sun is around 6500K, and a true full spectrum tube will have a kelvin number close to 6500K. A lower number means there is more emphasis on red ["warm" light is more reddish], and a higher number blue [blue light is more "cool" in appearance]. Light with the colour balanced around 6500K is referred to as "daylight" or similar terms. While blue is the most imortant colour for aquatic plants, it makes the aquarium look "cold" and rather spectral. You need to balance with some red (the second colour plants need), and then with green to make the colours appear natural.
Marine tanks where corals are growing need strong blue light, but this is not suitable for freshwater plant tanks. I'm not familiar with your present NEC Biolux tube, but from what little I can find out online it seems to be a good tube for planted aquaria. A similar tube is the Life-Glo (the Life-Glo 2 is a bit less intense, the Life-Glo seems closer to your NEC in intensity) or Zoo Med's Ultra Sun.
For your 70g (assume it is like mine, 4-feet length) two 40w tubes [Zoo Med are 32 watts I think, doesn't matter] are better. You can see in the photos of my three aquaria all the plants that will mange fine with this light. The nice thing about two tubes is being able to accent blue with one of them and still have a good balance with the other. MoneyMitch's recommended pairing is a good option. Or one Life-Glo 2 and one Phillips Daylight Deluxe (you can get these at Home Depot for very little money) which is what I currentl have on my 70g and 90g tanks.
On my 115g I have one Life-Glo 2 and one Lightning Rod Ultra Daylight which is 11000K so a blue accent. It is noticeably more "cool" but it works with the fish and plants in this tank. The other two are just ever so slightly "warmer" in appearance.