I'm funny, I'm a nut to research everything I do to death, but in the end, I'm not a stickler for rules. If I was satisfied with the sound, I would leave the speakers with the 5' distance on either side of the TV. Your only 8 feet back so although it's not an equal distance, it wouldn't be enough to bother me.
I think having one speaker way off to the side and one close to the TV would skew the soundstage a lot more and it just wouldn't look aesthetically correct to me, personally.
I definitely wouldn't be ambitious enough to move those big tanks unless the sound wasn't working at all for you.
I'd love to see some pics of your tanks. I love salt water aquariums but out of respect for the fish, never tried to do one myself.
