You can't add anything to that tank without effective filtration and without cycling the tank to establish the right bacteria to remove the ammonia that the fish produce. Remember that these fish are secreting ammonia and when the ammonia gets to even a relatively low concentration it will start to kill the fish. The filter isn't just to keep the water looking clean it is the main place that the the the biological filtration (conversion of ammonia to nitrite then the nitrite to nitrate). The gravel is also important for the biological filtration.
If you haven't got filters and a cycled tank the only way to stop the ammonia killing the fish is to do almost daily water changes.