The size of the tank sounds OK for 3 ACF but 79F is pretty high for X. Laevis, it could be the problem. They do better at ~70F.. if you have a heater you should turn it down or remove it entirely as long as the water stays 68-72F.

Higher water temps means their hearts beat faster, I don't believe X. Laevis does well at 80F+ they are not tropical animals, it puts strain on their hearts which could lead to a stroke.

Run a water test for nitrite and nitrate.. you should have 0 ammonia, 0 nitrite and under 20ppm of nitrate if your tank is cycled correctly.