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To answer your basic questions:

Lumens is the measure of usable light the bulb puts out. The more lumens the brighter the light.

The "best" K (effective color of the combined spectra put out by the bulb) is a good way to start holy wars. I'm sure someone will correct me if I'm wrong, but I understand that anything over 5500K is considered "getting there". Anything over 10000K is too blue, and the plants can't use the light as efficiently (those bulbs are more oriented toward salt water critters).

CRI is the color rendering index. It's a measure of how well a standardized color matching card matches what it looks like (or more importantly photographs like) in full sunlight. From my extremely limited experience, under water the rated CRI doesn't really track - the "stuff" in the water changes the characteristics of the light. I've used 6500K bulbs with a CRI of 91 (really good color matching), and the fish and plants look nice. However, with the 9325K, CRI 61 bulbs I'm using now, the neon colors of the various Tetras really pop. There's a noticeable difference in colors if I go from one bulb directly to the other, but they both look really good. I use the 9325 bulbs (compact fluorescents made by GE) because they give me a bit over 2 watts per gallon, versus less than 1 with the "better color matching" bulbs.

From what the Big Boys and Girls say, if you can hit 2 watts per gallon, that's kind of a "good" starting point for putting just about any plant you want in a tank. OTOH, I've seen many tanks and many postings with people running 1 to 1-1/2 watts per gallon where the plants do really well. If you have lower light capability, choose plants that require less light.

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