The day after my Grandpa died, I went to his place and picked up his two fish tanks. I put them up on my fireplace mantle and cleaned them up. I knew pretty quickly I wanted to consolidate both tanks into one and I began planning.
The first thing on my mind was: is it actually okay to place a fish tank on top of a fire place? There is a lot of myopic discussion of this topic online regarding two possible issues: weight and heat level.
My fireplace mantle is 100% brick, I'm quite confident it could hold up much more weight than these two tanks.
Regarding heat, I did a couple fires with and without the fish tanks and attempted to measure any impact on the mantle with a infrared thermometer and a FLIR attachment for my phone. I concluded that the heat increase was negligable if there is any at all. I do not normally have huge fires in my fire place, usually it's 1 or 2 duraflames at a time, as decoration in the winter. If I am doing a big fire, it's because my furnace is broken or something like that. I did test a bigger fire and could not measure any concerning impact.