I believe the problem is some heat packs might get too hot for fish
for example, the ones from Dick's sporting goods say
Average Temperature: 127 deg F (53 deg C)
Maximum Temperature: 154 deg F (68 deg C)
Duration: 12+ hours
I've seen the type of heat packs designed for tropical fish say average temp 70 F and max 110F
Thats a big difference between keeping fish warm and cooking them