I have two 30 year old red eared sliders in a pond outside. I've had them since they were hatchlings the size of a quarter with their egg tooth still attached. They are now a good 11 or 12 inches across.
Turtles are very cool, just a whole lot of work. They are very messy, can be smelly, and can carry diseases like salmonella.
If you want to keep them in an aquarium inside, I recommend a bare aquarium (no gravel- becomes a nightmare to clean very quickly) filled about half full with water. They need a place to be able to get out of the water and dry off completely- a shelf or pile of big, smooth rocks.
Filtration needs to be way more than what you would have for fish. I like Aquaclear hang on the back filters. Get the biggest one you can, should be rated for an aquarium larger than what you have.
Light is critical. They need UV even more than our birds do.
Inside the house you probably don't need to worry about heat, they should be fine.
Some people keep a smaller aquarium as a feed in tank and put them in it to feed and move them back to the main tank after they poop, but I have never done that. Then you have two tanks to keep clean.
The water and filter medium will need to be changed often- maybe a few times a week.
Always always always wash your hands really well with soap and hot water after touching the turtles or their water.
Red eared sliders are carnivorous when smaller, and need more and more vegetable matter as they get larger. I feed Tetra Reptomin as a basic diet and supplement with snails and bugs from the yard and spinach, chard, kale, and aquarium plants.
I moved my turtles outside when they were about 5 years old and around 4 inches. They are now in a 150 gallon horse trough with a pond filter and a nice pile of rocks in our enclosed atrium (raccoons, opossums, birds of prey, dogs and cats can be a problem).
In the summer, we change the water and rinse the filter about once a week. In the winter (October through about right now), they don't really eat, so I just change the water every now and then. It doesn't get cold enough here for them to truly hibernate, but they do slow way down.
I don't know what kind of turtles you have available for pets in Australia, and red eared sliders are the only ones I have experience with. They are interesting creatures and cool pets, but are A LOT of work, and they live as long as big parrots (my daughter has said she will take them if they are still around when I die- I'm 50 now, so no telling who will go first, me or the turtles, lol).
I have also had aquariums since I was 12. I have had fish that lived for years and years (11 years for one clown loach, lost him due to a severe power outage last fall). If you are loosing fish at the rate you mention, a couple things come to mind...
Every two weeks is probably NOT often enough for water changes. I change 50% of the water once a week (85 gallon tank- 60x18x18), and rinse the filter medium. You should never have to change all the water. Learn and understand the nitrogen cycle. Get a liquid test kit for ammonia, nitrates, and nitrites- without this you are guessing. Natural, biological filtration takes awhile to establish in the aquarium, but once it's working maintenance becomes pretty routine.
Make sure the fish you have a re compatible and not stressing each other out, don't over crowd, and don't add too many fish too quickly.
I keep the water temperature between 78 and 80 degrees (Fahrenheit), which is warmer than generally recommended, but over the years I've learned it works better.
I hope there is something useful in this long post!
