It would be plausible to me that you have a few problems. I cannot really comprehend a SW tank without a heater in Calgary, AB. ( Seeing as I keep coldwater crayfish in a tank at room temperature). My house is certainly not kept at 80 degrees, but with evaporation, my room temp tank is about 68 degrees in the AM. 2 degrees C is a major fluctuation.
I believe you said the last clown died the same day you added the sand? If this is the case, it is also possible you suffocated the life in the existing bed, thereby causing a spike. Although it sounds like the clowns were in stress before that.
Alan made a good point about the frogspawn, and what were your thoughts on that?
You haven't commented on SG or pH, or provided any specific values for the paratmeters you did comment on. Some peoples idea of "perfect" is indeed NOT.
Good luck.
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