There has to be food to fuel the bacteria. Without it, no growth. I have used brutes, rubbermaids and some of the white ag ones for years without any slime build up. I mostly use IO salt, but have used some Tropic Marin and Salinity lately.
Basically, with the glass, you are putting it all into the tank rather than leaving it in the barrel. The issue is that the bacteria (or diatoms) can grow, so I would not be too concerned if they grow on the glass, but how to make sure that what is fueling them doesn't enter your tank to fuel other things there.
If you used a slime-common salt a few times, then you could get the bacteria for many, many water changes after that... unless you cleaned with a real cleaner. Clean that thing out with some real acid and try a different salt (IO, RC or Tropic Marin) and see how it goes... my guess is that you will be all good.