I once had a freshwater catfish and I noticed over a couple days it had become a lot more active. On about the third day, I came in and found him floating dead. He was over a foot long. I hated losing him. I discovered that my heater had malfunctioned and had let temps go up really far. The only fish that survived were Oscars. Now, when I see a sudden increase in activity, I check temperature first. I also have learned that most heaters, when they go bad, stay on, instead of just turn off. I now use a few smaller heaters instead of one big one. That way if one sticks on, the others just stay off.