72°F will not hurt them, granted it's at the lower end of their comfort zone, but I highly doubt temperature is your problem. I've had the temperature drop in my tank to 58°F for a couple of days due to extended power outage and didn't loose a single adult or fry, granted their activity level dropped dramatically until power was restored and temperature returned to the normal 74°F I keep it at. If you are new to this you are probably making many of the mistakes that we all do until we learn better. As a result your fish may be living but are probably not at optimum health which will result in infrequent births and high mortality rates. I suspect a couple of things, if ANY of your fish are relatively new you may have disease in your tank, also if your water quality is poor you may have disease. If water quality is in question change it. Clean water goes a long way towards fighting and preventing fish disease. I am a firm believer that you can't do too many water changes cause after all fish in nature get continuous new clean water. Check your PH, Ammonia, Nitrite and Nitrate levels. The temperature being at 80°F now will assist in curing many diseases also a small amount of aquarium salt added will also help. Another possibility is that the mother was not quite fully mature and as a result the fry did not develope properly, if this is the case there is little you can do except hope for better luck in the future.