Electrical help required in DIY LED circuit

[lifesci]

I'm gonna keep this short, so if any additional detail is required, please let me know.

I want to run multiple LED's on one driver. The driver I'm looking at is rated at 100W, and the LED's at 20W each. The supplier asks me what specs I want on the LED, i.e. what combination of voltage & amps (since afaik many combinations of volts*amps can give the same wattage, i.e. 50v * 1amp, 25v * 2amp, etc). How do I respond to this? I don't know how these different specs will affect driver output. I'm guessing there is some degree of math involved. Any help here would be greatly appreciated.

Secondly, my knowledge on electrical jargon is very basic, so I have a few questions.
1) The driver provides constant current; how is the voltage regulated? Or does it not matter?
2) If I add a 6th 20W LED to the circuit, what happens? Does the driver output more voltage to compensate for the additional LED? Or are all 6 LED's underpowered?

Thank you very much.

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In case anyone is wondering what I'm doing.. I've built an aquaponics system that employs photosynthetic bacteria at the base for which I need specific wavelengths of light (and that can only be achieved efficiently using high-powered LED's). Its a research project at university.