Living room only has a single small window that can be opened, and we can't open it much overnight as we have an indoor cat.

Shut him out of the back of the house last night and opened those windows more and damn those feel cooler right now. But they heat up a lot in the afternoon while the living room is throughout the day the best at keeping out the heat. Just more difficult to remove heat from.. Also with a pretty big aquarium it's probably got a massive thermal mass from all that water.

Think we need to look into some kind of cat proof window covers. Ideally for 2 windows to get some airflow through the living room and some other room of the house. Chicken wire might be a fairly cheap option and I could do with getting some for other projects too.

If you have multiple levels (e.g. a house) open a window downstairs and one upstairs. The rising hot air inside will draw in cooler air from outside, but the hot air needs to be able to escape. If you power that with a fan, that's good too.

I've tried that, it doesn't work. I must have a secret furnace in my room somewhere. Once it gets to about 9 or 10 am it's better with the window closed and the door open, since the hallway always feels a lot cooler. Eventually there'll be a breeze that will run from the north side of the house to my room and that will cool things down. In the meantime, a spray bottle and fan are my best friends for the next few days.