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For a fowlr you could go from 6500k to 10,000k and it will look great. GE makes an inexpensive one that is sold on Drsfostersmith.com pretty inexpensively.
20,000k are just at the bluer end of the spectrum. They will give your tank a bluer look and you will not need to add blue actinic bulbs to your setup. I have 20,000k's on our tank and it is beautiful. These are very good if you think you might go to a reef someday. The 10,000k's are a whiter light but you would need blue actinics also to get the right mix. However if you are just FOWLR right now then it's just a matter of personal preference.