Water Hardness

The hardness of the tank water should be whatever comes out of your taps. Yours is soft, which is perfect for neons. It can be made harder by the additions of special salts but I wouldn't do that with the fish you have. Maybe if you wanted hard water fish, but not neons.

I see you are doing water changes only every 3 to 4 weeks. This could be a problem. You have soft water (GH of 100ppm) so your carbonate hardness (KH) is likely to be low as well. Rather than go out and buy a tester for KH, ask a shop to test a sample of tap water for KH (assuming they do that in Australia  ) KH buffers the water against drops in pH and it gets used up. If it is low from the tap and you do water changes every 3/4 weeks, there is a good chance you will suffer a pH crash, which is not good for fish.
Besides which, most people do weekly 25%+ water changes and gravel cleans - and this will prevent pH crashes. I have low KH (54ppm, 3 german degrees) and I've suffered pH crashes when I didn't do enough water changes.


As for stocking, it is impossible to say any tank will hold x number of fish. You have to go by the size of the fish and the behaviour of the fish. Neons should be fine in 35 litres as 10 US galls is the recommended minimum size. But a few more would be a good idea as they like to be in shoals of 6+.
The zerbra - if that's a zebra danio, it also needs to be in a shoal and a bigger tank. They may be small fish but they are very fast swimmers, being able to cross a 120cm tank in around 1 second. This is what I meant by taking behaviour into consideration.
Half a dozen neons put you at half stocked, so you do have room for a few more fish. I am well aware that there are fewer fish available in Australia compared to the UK so you might no be able to source my suggestions.
A pair of peacock gobies/gudgeons (they go under both names); or a pair of scarlet badis; or shrimps.


But.....
I see from your post in the introduce yourself thread that you've only just got the tank. Did you do a fishless cycle? If you didn't, don't get any more fish until the tank has cycled.




One last comment, you'll find that fishkeeping has changed a lot from when your father had fish. Listen to him, but check to see if his methods are out of date. For example, doing few water changes was the norm a few decades ago but we now know that lots of water changes are better for the fish.