This stuff, or similar.
If you think the diesel is going stale, you can either empty the tank and store the removed diesel, adding it back in to fresh fuel at, say, 1:4 ratio (1 part old to 4 parts new). This assumes that you would go through that amount in six months or so.
If you have a big fixed tank and barely use it, you might consider emptying the main tank and fitting a small tote tank with a changeover valve between the main and the tote on the fuel line to the pump. That way, your diesel is always reasonably fresh when you are not using the engine a lot (winter race series