Earlier in my attempt at Koi-keeping, I was fortunate enough to purchase an excellent pond heater for one-fourth the original price. I saw it advertised on Craig's List and called the seller to come and look at the unit. After a shaky sale (that should of been an omen to NOT purchase). I went on my way home to set up the heater up on a 250-gallon QT tank on the side.
The heater was a wand probe type, with a plastic screen that surrounded the heater probe so as not to burn the fish if they ventured too close. It had a shut-off float, a controller, and a four-foot wire thermostat wire. When I first gazed at this piece, the thermostat, of the heater, it seemed to me to be slightly flimsy; Always listen to your initial impressions. Yes listen.
A few weeks later, I worked my local koi club's koi show. I was able to find, in the vendors area, a very nice Ki Utsuri, two nice Kumonryus, and a kohuku. I brought them home and floated the bags in the tank to get the water temperatures stable. Since the current water temperature of the tank was lower than I wanted the fish to exist, I slowly went about raising the temperature. I increase it by two to three degrees every other day.
The process went along smoothly for the first week. I' d adjust the control in the A.M. and come back that evening after work and check on their status. On the fifth or sixth day of this routine, my neighborhood had higher than normal winds blow through. On my ride home, I notice much debris scattered on the streets. I had gotten the temperature in the tank by this time up from 60 to 74 degrees.
When I came out to check on the new purchases that afternoon, the first thing I noticed was a light tree branch had snapped off and fallen near the tank. Attached to the branch was the heat sensor cable, exposed to the air. The next thing was the oil-slick appearance that seem to be covering the surface of the tank. Then I saw the floaters; all of my newly required fish were floating dead on the surface.
I read the thermometer that was in the tank; one hundred and two degrees. I had set the control for seventy-four; the wind had blown; the branch had fell; the sensor became exposed to the air (which happened to be in the low 60's); the heater turned on; the temperature never reached; the heater kept on; the fish stewed right there in the make-shift Jacuzzi.
I now have secured a clip-on point inside that tank so, I am hopeful, this won't happen again.