I'm thinking 78- 80 degrees isn't enuf to melt a plastic that thick. I forgot what type my heater was, but it was glass with the plastic netting type material around it. My powerhead was prolly 2-300GPH with a 3/4", 6ft hose attached, just thrown in there pointed at the bottom to ensure the salt got mixed well. Would sit like that for days and weeks.
P.S. the 6ft hose was so I could just reach in there, grab the hose and pump water into my tank.
"Life may not be the party we hoped for but while we are here we might as well dance!"