You are correct in that you probably did not cycle it long enough. When the bacteria start to form nitrite from ammonia, this is when it gets extremely lethal to your fish. 2 days of waiting is insufficient to be certain of the fishes' conditions. Only when the nitrite is at zero and there is nitrates being produced is your tank ready for any additions. You should never add a lot of fish all at once, but a few at a time. Proper acclimatisation of the fish helps to reduce the stress on them. If you just dump them in and the water parameters in your tank is vastly different than that of the LFS or the water in the fish bag, they will die from shock in some cases.
I cannot see your picture but there is probably too many fishes for the initial introduction. You should always clear up uneaten food or else they rot and contribute to the overall deterioration of the conditions in the tank.
You do not need a thermo-controlled heater in our climate. They are only useful for certain species like discus, otherwise are not needed since we do not experience cold conditions here.
Adding the bacteria enzyme can help, but chances are, some of the fish may die in this period. I hope you are prepared for this eventuality, should it happen. Good luck though.
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