I notice from your other thread that you have had the tank a week.
You really need to buy your own test kit. You are doing what is called a fish -in cycle. Until you have your own test kit, you need to change at least half the water every day.
And ignore what I said about getting more neons, that can wait until after the tank is cycled. More fish will make more work for you during cycling.
Fish excrete ammonia - it's their version of urine. But ammonia is toxic to them; it burns their skin and gills. In a cycled tank, there is a colony of bacteria which eat this ammonia and they turn it into nitrite. Nitrite is also toxic; it binds to their blood and stops them taking up oxygen. In a cycled tank, there is another colony of bacteria which eat nitrite and they turn it into nitrate. Nitrate isn't nearly as toxic and we remove it by doing water changes.
But in a brand new tank, there are no bacteria; they have to grow and it takes several weeks. Firstly ammonia will build up in the water and the fish keeper must do water changes every time the level is higher than zero. This means testing every day, which is why I say you need your own test kit. Once the ammonia eaters start to grow, they start making nitrite. Water changes need to be done whenever nitrite is above zero.
At first there will be only ammonia and you will need to do daily water changes to keep the level at zero. After a week or so, the ammonia starts to be eaten and the level isn't quite as high, then it slowly drops till it stays at zero. But at the same time, nitrite starts building up faster and faster so again daily water changes need to be done to get it down.
After a few weeks, you'll find you don't need to do water changes as often and one day you'll realise that the levels have stayed at zero for a week. The tank is then cycled and you need to do only weekly 50% water changes.
There are things you can do to help.
Get some Tetra Safe Start. This is the most highly recommended bacterial starter.
Feed the fish only once every 3 days. Less food = less ammonia.
Get some live plants. These use ammonia as fertiliser and will remove some from the water. Even just a couple of bunches of elodea left to float will help.
Use a dechlorinator which detoxifies ammonia. The effect wears off after 24 hours, but it will keep your fish safe until the next day's water change. Seachem Prime also detoxifies nitrite for 24 hours.
Yes, neons are soft water fish and guppies hard water but at the moment, keeping them alive is more important. Once mehifish is on top of cycling, then we can discuss hard/soft water. There is a lot to take in at the moment.