In a tank that small with no filter, you should be adding something like Prime and probably changing the water daily. You also need a heater. Somewhere more experience can tell you how to treat the damage already done.
Prime only locks up ammonia for 24 to 48 hours max,I'd recommend getting a water conditioner like Prime. It removes harmful chemicals in the water & locks up Ammonia
I do not recommend putting shells in a freshwater aquarium.I read once that they should not be used in freshwater tanks due to particulars or calcium going into the tank or something to the effect it might raise the pH
Most bottled water is NOT fish safe, Spring water can vary in composition, in terms of minerals and pH, again creating an unstable environment. Bottled water can even be chlorinated. Combine this with the instability and you're better off using dechlorinated tap water in most cases.What additives do you use? Boss using bottled water, no additives