Can I use light from fish tank to grow orchids ?

[goodgollymissmolly]

Newbie, hey you're ahead of the game here because you already know about light intensity and spectral output. You are correct that plants use red and blue at the wavelengths you specified (and Ray's page shows) and plants are green because they reflect green. I suspect that you also know that the Kelvin temperature of the incident light is meaningless to the plant (though it has a lot to do with what you see when you view the plant). There are an infinite number of wavelength combinations that will appear as the same Kelvin temperature but lack significant portions of the spectrum. That is known as metamerism.(The characteristic of the eye that views various spectral combinations as identical).

The one place that I think you might be going wrong is that I think you are suggesting that plants can utilize red or blue rather than red and blue. The two types of chlorophyll use the two wavelengths independently. The plant needs both. I have no idea what you will find out regarding how much light intensity you have external to the tank, but obviously it's going to be blue. If you have enough intensity, you might be able to get the red by hanging an incandescent light nearby. Just an idea.