As stated above you need to find out all those parameters in the tank and optimize them for the guppies.
I agree that a good filtration system will solve most problems.
A tip with guppies, try to buy locally bred stock. Imports often sit in solutions of tetracycline at the exporters, if you want to know more about this then pm me.
The only thing I would disagree with is using carbon. 'Purigen' is light-years ahead as a chemical filter media and you never have to buy more because you can recharge it.
I think it would be very risky not to use water conditioner/ager, all tap water has chlorine and chloramine in it, at low levels it will kill the bacteria in your filter and the bacteria is the whole reason to have a filter, at higher levels it will kill the fish. I suggest using 'prime', it also binds ammonia and nitrite to neutralize their toxic affects, this might just save your fish as it did my lungfish one very distressing evening. Also if Adelaide's tap water is anything like Brisbane's, at different times the tap water itself will have high levels of ammonia and nitrite so it is just much safer to use prime.