Indeed, it does sound like you're trying to refuel from them when they haven't harvested anything. remember they probably only have a few sorium harvesters, so they can't harvest much at all-- my fuel harvesters have about 10-20 each and they still only harvest a pittance per year-- and a large slow tanker would probably have at least a million's worth of fuel capacity, which will drain the entire harvester in moments. Also having a colony on Io for a fuel base is a bit too close. I remember one of my first errors-- making a colony on luna and setting my colony ship to cycle dropping colonists off. Luna is so close to earth I had 5mil+ colonists on it dying of starvation before I could import some infrastructure because they could make the round trip so fast.
If you just get errors popping up when you try to select your tankers just keep clicking em away, but for 7-8 tankers it's gonna be quite a lot of errors unless they're in the same TG. :3