Doug,
A 7x16 tandem would be a good choice, as long as you can fit everything you need to on it, which I think you probably can.
The thing about a tandem axle is that when properly loaded, the weight is spread across the axles better, and puts less weight on the tongue. Also, a tandem will tow easier, and be less squirley than a single axle.
As far as leaf springs vs. torsion axles, I have both styles, and I think the torsion style may be a bit more stable than the leaf. Also, I notice that the leafs tend to squeak, which annoys the hell out of me sometimes.
If you choose to keep the single axle, you would be best to not fill the tank, maybe half at most. Of course, if the tank isn't baffled inside, you're going to get a lot of water sloshing which makes the trailer more squirley and harder to stop.
Hopefully you have trailer brakes, if not, I could not recommend using that trailer at all. Some will say it's ok to run without trailer brakes, but the fact is that most states require them at weights above 2500 pounds, not to mention it's rough on your truck brakes stopping that much weight. You'll find out the first time someone cuts in front of you and takes away your stopping area.