I have used a floating tank heater for years--currently in my 75 gal. galvanized oval trough, but in larger ones in past. In a metal tank they don't need any sort of 'cage', which IS needed for a floating heater in a rubbermaid or similar, tank. I use the red one(made by API, I believe? it's the only kind I've ever had...), and TIP: get the 1000 watt. The 1500 watt is not necessary unless you live at the North Pole, or have a humongous trough!--- and uses lots more electricity!
I run the cord through a piece of HD PVC, with a rt. angle 'joint' just above the heater itself, so that the cord is essentially fully protected. You do have to be sure that the water level doesn't get too low; that *can* leave the heater 'dangling' above the water...though the kind I use IS supposed to shut off if the element is out of the water. My horses generally don't bother the heater; if you have some that would, you might 'have to' use a sinking type.
We ran electric out to the large pen w/ large run-in shed where the trough is, so I am able to do without an extension cord. TIP TWO: on the electrician's advice, I use a programmable timer, and plug the heater into THAT. We have TIME OF USE electric rates here, and using the timer to 'match' the 'offpeak' rates(may have to reset if it gets REALLY cold), saves a lot of the cost of HIGH PRICED electricity(On-peak is $0.15/Kwh here, and I have an all-electric home.)I use a HD timer w/ a grounded plug, plugged into a grounded outdoor outlet--AND, I put a waterproof cover over it the whole deal. I have ONE goldfish, with me now for about 9 years, who lives in the trough, and LOVES the heater, BTW!
Now, in the individual pens, I LOVE the heated buckets. I use one HD outdoor extension cord w/ a multiple 'head' to plug in TWO buckets, outside during the day, inside in bad weather or really cold nights...and again, I use a timer.
Margo