That is interesting. I had a 100 gallon freshwater tank(and eventually satlwater) across the room from the insert for over 20 years. It was slightly off center and not directly infront of the fireplace. Then last winter, I had a 120 sitting about two feet further back than the 100, so it was not infront of the insert at all but it was in the room. The 100 gallon never got that warm and I struggled to keep the 120 as warm as 77. It may also be a contributing factor that the room has a cathedral ceiling. I was concerned about putting one beside it though just because it was so close. Maybe your insert is rated for a larger area than mine.....
It probably is very room dependent and type of fire dependent. I know it's summer, but maybe launch up a fire and see how hot it gets in that spot.
My fireplace really barely puts out heat. It's mostly decorative. Back when I had one I used for heat, it would get kind of warm in front of it, but not really at all off to the side of it. Clearly, I had a different set up than
@StirCrayzy .
I have a tall ceilings like you.
I know that *I* wouldn't be concerned at all putting my tank right next to my fireplace off to the side. In fact, I thought of that before. In part I didn't just because in my particular room it made that side of the room look "too busy", but heat was never even something I was worried about. But, clearly our experiences vary. I can't think of anything to do but light it up and see.