I always buy old homes and all will have a heat problem in summer especially here in New Orleans. Give me a attic fan and a floor furnace I'm happy. Just the way I was raised ! NO a/c really needed with the way homes in this climate are made. Look at from a different point of view , yes you can keep trying to cool just the tank but...
Here in the US most windows can open from top or bottom. Always slide open the top that's were the heat goes up. Window fans help but only if placed in upper section of window . If place in lower section you are sucking out the cooler air , place as high as you can and get the hot air that rises out. The old way here are windows that go from floor to ceiling. Summer open the top and winter open the bottom. You want it to pull hot air out never blow more air into an old or hot home.
Window film with UVA protection helps a lot . Comes clear or tinted. Some comes as a static cling very easy to work with.
Use a UVA backing on curtains helps. My very old house has several 12'x6' picture windows in room with Piggley. It's Hot in summer! I can't sew so I made my own backing with UV protection for curtains more like a window shade. That alone helped greatly! Got the room down 5* on it's own. I improvised material with this , comes in different color's , patterns and prices just shop around :
https://www.amazon.com/Pacific-300081-1 ... protection
It's sold a patio shade covering. Sure you won't need a 100 foot roll as I needed , will be cheaper for you if one or two windows in room. Just make sure it's a type with UV protection there are several types out there .
Or you can buy UV protection shades already made ! The size of windows I have was cheaper to make my own and I did the whole house.
If you own your home and in a hot climate "next time" your roof needs replacing go with a lighter color , black /dark colors absorb more heat , it makes a big difference !
Get creative on the way air moves throw your home , use cooler air from an cooler room to pass into hotter room.
Here the "old way" was/is to have summer and winter screens on windows to!
Better to cool all the room/ house since the tank takes on the temp of it and makes it nicer for you too !
If you try fans be careful of drafts on tank , turtles catch colds even in summer time from them.
Now if you are just rich you can have the house sealed 100% with a fresh air return system go that way but $$$$$ ! There are some very Hi-Tec models that do work great ! But the cost ?
Just a note 30c is not that hot that's 86* f almost the temp's I started with and got the room down to 76* with no a/c using above ideas !
If above still too much try thicker curtains with UV shades and cheap foam board cut to fit tight in windows for the summer , that helps too ! Just making room darker can help.
If you have and use an a/c , even a window unit , the film and or foam board and thicker curtains does help. Never try to shade block an a/c unit it uses air movement with all the vents on it to run in the heat. Window awnings will make the unit run hotter shorting it's life and will not be as efficient. A tall tree / bush several feet above unit could do better not affecting the air movement around the unit itself.