In a couple of other threads I've answered this, but I'll summarize it here to save time. I agree, the so-called "plant" tubes turn the tank purplish and the colours of the plants (various greens) do not appear natural, nor do the fish colours.
Light is made up of colours known as the spectrum, the "rainbow" you see when white light is shone through a glass prism. The spectrum graph printed on most aquarium fluorescent tube packages, and on their respective websites, shows the intensity of the spectrum colours produced by the tube. Accenting various colours makes the light different, so tubes that highlight the red and blue cast a purplish tint. Tubes high in blue tend to be "cooler" in colour, whereas red highlight produces "warm" colour. The colour is stated as degrees kelvin, and the kelvin rating of a tube is one indicator of the kind of light it will emit. This also depends upon "spikes" in certain colours.
All plant authors I've read agree that full spectrum light is the best. The sun has a colour temperature approximately 6500K at mid-day. Full spectrum light with a K rating of around 6500 best simulates this light. Plants require light in the blue colour most, then red; they reflect green but it is the green that balances the blue and red to give a natural appearance. Full spectrum delivers these basics.
I have experimented with several different tubes over 12 years, and I find the best is Hagen's Life-Glo 6700K full spectrum. On a tank with only one tube 9smaler tanks) I always use this tube. The Life-Glo has a more intense light, and the Life-Glo 2 is the same but slightly less intense. The Zoo Med Ultra-Sun tube is very similar. The Zoo Med Tropic-Sun is warmer (i.e., more red) at 5500K. I've also had success with the Phillips Daylight Deluxe at 6500K.
On a tank with two tubes you can mix the types, as I have done on my 70g, 90g and 115g. I use one Life-Glo on each. The second tube on the 115g is a Lightning Rod 11,000K Ultra Daylight tube. On the other two the second tube is presently a Phillips Daylight Deluxe, though I intend to replace one of these with a Ultra-Sun. The Tropic-Sun added with a Life-Glo 2 adds just a touch of warmth from the red and looks nice over tanks depicting tropical swamps. The Lightning Rod I use because it is higher in the blue (the higher K rating) and is said to better penetrate the depth; light penetrates water slower than air, and blue is better at penetrating water than the other colours (and plants require more blue) but the Life-Glo 2 full spectrum keeps this in balance. But the effect is slightly more "cool" that it would be with a Tropic-Sun and the Life-Glo 2.
One other consideration is CO2. I have never bothered with added CO2 as I obtain the look I want without the extra cost and bother (you can see my tanks in the photos under my aquariums). Plants require a balance of light (that must be of sufficient intensity and duration to match) and nutrients (being CO2, macro-nutrients and micro-nutrients). Light should always be the limiting factor. If these are kept in balance, at the level required by the plants (type, and quantity) in the aquarium, algae will not be a problem. Algae only takes control when the balance is broken. Without CO2, regular fluorescent light (T-8 is that, like I have) should be at 1-2 watts per gallon [your two tubes will probably be at this level]. Liquid fertilization ( a comprehensive fertilizer) added once or twice weekly will ensure all the nutrients are provided, the fish and other biological processes providing the CO2. If you intend to use added CO2, the light should be 4+ watts per gallon to balance, and more fertilization will be required. The duration of light depends upon the plants, nutrients, amount and type of light; generally 8 hours per day is considered minimum. I have my tanks lit for 13 hours, there is 1 watt per gallon or slightly less, and I fertilize twice a week.
Lastly, the type of plants is important. Stem plants tend to require more light (they grow faster) and correspoinding nutrients to match. Some will do well at lower light levels (liek the Pennywort in my 90g). With less lighting (1-2 watts) rooted plants like swords and crypts will grow well, a bit slower, but be healthy and thriving. Low light plants (Anubias, Java Fern, mosses) and floating plants will be fine.
Hope this helps.
Last edited by Byron; 08-16-2009 at 11:26 AM..