First of all welcome to the forum and to TFK
Now, with purchasing a new tank and getting fish in right way, your tank is likely cycling.
Cycling is the process of "creating" good bacteria needed to break down fish waste. You can read more about that here. A Beginner's Guide to the Freshwater Aquarium Cycle
The test strips you have purchased, sorry to say are virtually useless, they do not give "true" readings. It would be highly recommended to purchase a liquid test kit such as the API Master, these tests are more accurate and will give a better picture of what is going on in your tank.
I also have to point that NONE of the fish you have are suitable long term for a 20g tank
You can check out our profile section at the top of the navigation bar, Take for example the Rainbow shark, you will see that it is highlighted, this means it is available in our profile section. You will see the suggested minimum tank size is a 4ft (48") tank, this is because the fish attains 6 inches in length when adult.
Now you have the fish though, lets work on saving the ones you have. You are going to have to perform DAILY WITHOUT EXCEPTION water changes of at least 50% of the tank volume. Replace the water with tap water treated with a water conditioner (there are many available although Seachem Prime is very good one). Try also as well to get the replacement water as near to the tank temperature as you can. This will prevent the fish being shocked from the addition of very cold water.
The additions of live plants will also help.
What is happening in your tank at the moment is the Nitrogen cycle is starting off. Fish waste and Carbon Dioxide from their breathing are being converted from Ammonia --> Nitrite --> Nitrates. Ammonia and Nitrite are both extremely toxic to fish and is what likely killed yours. The need for daily water changes will help remove the toxic effects of these. The good bacteria need to build up in enough quantity to be able to handle the fish you have. NEVER clean the filter out with tap water, this will kill the good bacteria, if you need to clean the filter, then when performing water changes, wash it in old tank water ONLY.
I think I have covered quite a bit here but at the time of writing this (late at night), I may have missed something. Perform the water changes, get a good liquid test kit (about $20, if you are in the US?). The purchase of the test kit will help immensely.
There are also some issues with your water hardness and pH will the fish you have but messing with these is NOT RECOMMENDED even if you are told by a fish store employ. Unfortunately many fish store employs know very little about the fish and their proper care, they are there to make a sale and money for the company / store.
Please DO NOT be disheartened by the information here, we will try as best we can to save the little guys and give you some fish to enjoy until such time as you can get a bigger tank or we can offer suggestions to fish more suitable to the one you have. Do you know if your LFS (local Fish store) will take fish back at all?