Shoot in RAW. Set color temp during post. It's the easiest and most color accurate way.
That's what I do, and I'll generally just eyeball the white balance to get it right. I think shooting in raw is definitely critical. Assuming you've done that, a couple other tricks for getting the white balance correct in post processing are to use the eye dropper white balance tool (most any raw editor is going to have that) on something white that's already in your tank. . .like the sand. Keep in mind, though, if it has any color in it whatsoever, the result won't look right. The other, better way is to put either a waterproof white balance card, or something like a (clean) white bucket lid in the tank, tilt it about 45 degrees so it's getting lit by your aquarium lights and take a picture of it. You can then either set a manual white balance in your camera and continue shooting with that white balance or use the eye dropper to white balance that object in post, and then use those white balance settings for the rest of your pictures. If you're going to do the latter, however, do
not set your camera to auto white balance. You can use any other setting (fluorescent, shade, sunlight, etc.) since you'll be correcting the white balance anyway, but this technique will not work correctly in AWB, since the camera is potentially using a different white balance baseline for each shot.
Also, simply setting the color temperature to 20K in post probably won't work. I've never tried it, but just because your bulbs peak at 20K Kelvin (which is all that rating means), doesn't mean that's the white balance setting, especially accounting for the effect of the water and any color shift in the bulbs themselves. But I could be wrong. . .
