Hi,
I haven't read everyone else's advice but I used to work in a pet store/aquarium and take care of loads of fish. I am not sure of your experience with fish so I will just write as if you are a beginner and hopefully something will help you! Sorry if you are not a beginner - Maybe someone reading will get some use out of this too though
Some individual guppies are hardier than others and will survive in non ideal conditions - However, all different types of fish have different ideal water chemistry needs depending on where in the world they are originally from and what the water is like there -
Temperature
Guppies are tropical and like warm water - Doesn't have to be exact - somewhere from 24-28.
Water hardness
They also like 'hard' water and Australian water is 'soft'. To fix this you add salts to the water - Have you seen the blue salt crystals they sell in pet stores? It is a mixture of normal and epsom salts. If you go into your local pet store/aquarium they will tell you how much to use for your tank size. (You add more for guppies than you would for gold fish or other tropicals like Angelfish or neons). ** Water naturally evaporates out of your tank but the salts stay behind - so if you are just 'topping up' instead of taking out and adding more water, there is no need to add more salts.
Water pH
Guppies like a neutral pH as close to 7 as you can get it. Our tap water has a relatively neutral pH so the more often you do water changes the better your pH will be. Fish waste products are acid so the pH will naturally drop over time without a water change.
Chlorine in tap water
This kills fish and burns their skin - if not straight away, it will still lower their immune system allowing other things like bad water chemistry and/or usually non problematic diseases to kill them (Wrong pH and water hardness also causes the same immune system failure). There is cheap liquid you get at the pet shop that gets rid of chlorine- or else you can leave buckets of water in the sun for a few days and this will do the same thing.
Good Bacteria & water changes
One of the biggest killers of pet fish is people cleaning the tank too much - Good bacteria grows SLOWLY on all the surfaces in your tank - this bacteria is what breaks down all the toxic fish waste (ammonia>nitrites>nitrates) into neutral harmless products. Every time a person tips out all the water and washes everything down they kill all the good bacteria so then when the fish makes waste, it hangs around in the water and makes them sick and often die - This is why so many people can't keep goldfish alive very long!!- Ammonia lowers the immune system and also 'burns' their skin - It takes about 4 weeks to grow the bacteria back but by then if you have a lot of fish it will be too late and a lot of people may have done a complete wash again by then anyway.
The best thing to do is to only take out 1/4 of the water every time you do a change - using a gravel vac/siphon so you can get the waste out from under the gravel without washing off the good bacteria. These are cheap at the pet store and worth it! If you take out too much then when you add the new water it will be cold and have different pH and hardness to the existing water and it will shock the fish and also lower their immune system. Obviously the more often you can do small changes the better - Once a week would be a great amount but as time goes by you could do it less when there is more good bacteria present.
I really hope this helps, goodluck