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.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France