I would be concerned about smog particles and use Prime to treat for heavy metals, and since a roof will accumulate lots of particles you may have a heavy load of pollutants if collecting off of a roof. Especially if there is a long perod of no rainfall, with evaporation if that is possible in the barrel condensing the "stuff". And with long period of no rainfall like the roof, more time to collect dust, so the initial amount would likely have the higher concentration.
Think about how dust can accumulate on your car in just a week.
If you live near manufacturing or other industry which has possible pollutants, or near highways /roadways where rubber dust is constantly kicked up.
Off of the roof I wouldn't collect for use personally in a fish tank, it would make me too concerned. Unless it was after a decent amount of rainfall would clean off of the roof and then I may. I would use that kind of water for watering the flower beds, and use better water for the tanks.
If it is collected in buckets or containers I would be less hesitant, being mindful of what you live near.