Hi Mostafa
Welcome to Fish Lore
I'm really sorry about your fish.
Is your tank cycled?
Do you know the readings for
ammonia, nitrites and nitrates?
Here's some information about how to cycle a tank.
http://www.fishlore.com/NitrogenCycle.htm
Your tank is terribly overstocked.
The shark needs a tank of about 55g's.
http://www.fishlore.com/Profiles-BalaShark.htm
Your two frogs need at the very minimum of 5g's to themselves.
The mollies do better in a 20g tank.
http://www.fishlore.com/Profiles-Mollies.htm
The loaches, depending on the species also need a bigger tank.
Although the couple of kuhlis that you began with could survive in the 5g with nothing else.
Changing the water might help them for the moment, but there's no way around the overstocking issue except returning some fish/frogs or getting a larger tank.
Good luck.