Fuel Tanks 

First rule: always keep them full as this helps stop condensation forming in the airspace inside and then contaminating your fuel with water. Water in diesel will help the dreaded diesel bug grow.

To help protect against this consider using one of the diesel treatment additives that are available on the market. Also you can take a small sump pump and drop the pickup tube into the lowest point of the tank and pump out 2-500mls of diesel and water into a clear container. You will then be able to see if any water is present.

Second rule: Filter all diesel through a fine mesh funnel as this will keep most contaminates out of the tank and use silicon grease on the "O" rings of your deck filler as this will help get a good tight seal.

All outlets and inlets to the tank should be checked for leaks, make sure that your breather is not blocked and check that the tank is securely fastened down.