In addition to what Reef Bass has said, maybe try and put some cardboard or something in front of the lighting to block it out during exposure, and then crop it out in post to just show the front glass portion of the tank - you may get a more even exposure that way. Also, you said what shutter speed and ISO you used, but not what aperture... for FTS, you'll want something mid-range... f11 is a good starting point... maybe even try going into Av mode and imputting something mid range, set it to fire on the 2 or 10 second delay on a tripod, and also maybe try metering so you are underexposed by a stop... or even a stop and a half. That usually works. Also, no need to buy lightroom or trial it, you can do WB correction in Digital Photo Professional, the software that came with your Canon Camera. Just do a quick google search on DPP and white balance correction. HTH. Try again and repost!