WHY DOES MY HORSES????

I WAS WONDERING WHY DOES MY HORSES NOT WANT TO DRINK FROM THE 100 GALLON WATERER I HAVE FOR THEM? I HAVE CLEANED IT PUT IN FRESH WATER, BUT THEY STILL WANT TO DRINK FROM A LITTLE 35-40 GALLON BUCKET, I DO FILL THE BUCKET TWICE A DAY BUT WITH WINTER ON ITS WAY IT'S JUST TO HARD TO KEEP DRAGGING OUT HOSES TO FILL UP. PLUS THE 100 GALLON ALREADY HAS THE HEATER IN IT.I USE NO CHEMICLES TO CLEAN JUST A BRUSH. IT IS KEPT INSIDE THERE SHELTER BUT WHEN I PUT SOME OF THE WATER IN MY HAND AND PUT TO HIS MOUTH HE MAKE THAT FACE, YOU KNOW THE UNPLESENT FACE. I WAS GOING TO CLEAN AGIAN ON THANKSGIVING BUT IT JUST DID IT 3 WEEKS AGO. IS IT POSSIBLE THEY don't LIKE THE SPOT I HAVE IT IN?? HE USED IT MORE WHEN IT WAS OUTSIDE HIS SHELTER. AND I STILL HAD THE LITTLE ONE UP BY THE HOUSE AND HE USED THAT ONLY WHENHE WAS IN THE AREA. NOW THEY MAKES SPECIAL TRIPS TO THE HOUSE FOR IT. I JUST don't WANT NO WATERING ISSUES WHEN IT GETS REAL REAL COLD.

Personally I don't believe it's a "spot" issue. I moved my bucket around the property, and they obviously didn't care. What comes into mind is something with taste. What the material big tank made of? Any different from small buckets? Can the heater rust or something? Do they drink from there at all? Did you try to remove the small bucket and see whether they going to drink from big one?

Is the water coming from the same source? The water in the buckets coming from the same hose, well etc. as the tank? Is your heater/tank grounded? Maybe he is getting little shocks if it isn't.
Check all of these things and if everything is A Ok, I would remove the buckets and only give him the one water source. When he gets thirsty he will drink.