With a tractor what causes water to get in the fuel?

Diesel fuel naturally attracts water and you need to be very diligent and careful in keeping it clean. That starts with where or what you're getting the fuel out of. If the bulk tank is allowing water to get in from the outside, like rain, stop that from happening. Then you have to look at how well sealed the fuel tank(s) filler spouts are and replace any bad caps that could have bad seals. Replacing fuel filters on a regular basis is most important because a fuel filter will not stop water altogether. It's a wise idea to install a water seperator into the system between the fuel tank and the first filter,someplace where it's easy to get at so it can be drained on a regular basis. It all comes down to how well you prevent the problem from happening in the handling and distribution of the fuel.