I LOVE that aquarium idea!! So much juxtaposition, yaaaaas!
Besides that, I have to say, I love that fireplace! It’s so beautiful, and so many people would be jealous that you have such a cool, fun feature in your room!
I keep candles in my fireplace at the house, which still feels like a play on the intention of the feature, but in a much more elective and stylish way. It could be fun, also, to use the space to display fairy lights in a new way — maybe wrapping them around logs?! (Kind of like we did with this drift wood branch last Christmas!)
I love plants in a room, and two plants on either side of the fireplace could be so cute!! You have a lot of open space there, so you could get really creative with the height and shape of the plants you choose!