From the lower lethal temperature study they did when considering their invasive risk, they basically stop eating below 70 and 100% of them die at 61F. The USGS data indicates that temperatures in the caloosahatchee where it washed up get colder than that almost every year. It also wasn't anywhere near 7'. Looked like more like 3.5-4' to me from the pictures of the carcass. One of the original news reporters speculated it was 5-1/2 feet.
There is literally an Arapaima farm 25 miles from where it was found. It was no doubt an escape or a release. I'm not sure why no journalists picked up on that angle. Probably because they are kind of "off the map" and impossible to get ahold of.