5G tank is not very big, but for 5 to 10 guppies should be fine.
1. If your tank is first time to fill water in, you need at least one day to add fishes in.
2. Every week you need to changes at least 25% to 40% water, add water condition to the new water before fill the water in. You should check the amount you need to add. You can find the amount from the water condition container instruction.
3. Do not feed too much, they would not die if you only feed once per day. The amount should be they can eat all of them before they sink to the bottom.
Also where did you get your fishes? From pet store, sure they may die after a few day....
Need Guppies, send me 悄悄话.