I'd follow "rlitman"'s advice, for precisely the reasons he cites. . .
Further, I have to assume that in your application, you're storing a fair quantity of fuel on-site for a significant period of time, and very probably NEVER really draining the tank between top-offs. In which case, you REALLY need to look at getting yourself a fuel-polishing system
Wait, huh, what did I say?
My assumption was that the OP was either sharing the fuel tank for the diesel generator with a heating system, or perhaps he just has a small fuel tank that happens to be similar in size to a residential heating system's.
In that case, the fuel sales person probably didn't want to sell them diesel, because they don't want to empty a truck, and then refill it with diesel to make a 200 gallon delivery just to you.
If the tank is ONLY being used for a standby generator, getting ULSD delivered during an emergency may not be much of a problem. When your power is out, you're not the only one running a generator, and your fuel guy may already be going around the neighborhood with a truck with off-road diesel for just this. So you may be able to explain this to them, and get yourself on a route of diesel customer deliveries. Asking for an initial fill up, with the understanding that it is a one time thing may not even be that much of a reach.
If the tank is also being shared for heating, then expecting it to be topped off regularly with diesel is unrealistic unless you've got a tank of significant size (I know I have no problem asking for 500 gallons at work, but I probably wouldn't be able to ask for much less; not that less would be of much use to me).
As for "regulation", that's a state by state thing, but would be dependent on the size of your tank. Whether it is ULSD or HHO in the tank makes no difference. In NYS, the smallest tank at my work is 1500 gallons gross, and it is above the threshold that requires a permit.
As for polishing, I'm not so sure that's a great idea. It may be important in a marine environment where water is unavoidable, but in an aboveground tank that starts out with clean diesel, I'd start by taking a sample from the bottom of the tank. Check that for water and turbidity (cloudiness). There are sampling bottles where you read numbers on the backside of the label on the back of the bottle to get an idea of contamination. So long as the fuel is absolutely clear, and has no funny smell, I wouldn't mess with it. I made my own "sample thief" using a Mityvac hand vacuum pump, a 1-man brake bleeder bottle (to collect the sample), some tubing, and a weight to get the sample from the absolute bottom of the tank. You could also just use a stick with some water detecting paste on it.
If you find water on the bottom, there are materials that will remove it (a clean diaper would work too, as the filler is superabsorbant to water, but not oil). If there is no water, it is very unlikely you will get bacteria/algae contamination.
If you see cloudiness, then you need to fix that issue ASAP.
Anyway, it looks like you have a nice water separator and filter on that frame, before it goes through the engine's filter. Just remember that anywhere the fuel line is subject to freezing, it must always be routed with the slope down to the tank, so there are no places where water can collect and freeze.
