OH is a plumber and has told me absolute horror stories about rain water systems here in NZ. Typically, they are not filtered. And many times, the tanks do not even have a lid.
He has had to fish out dead bloated rats (and many other things) from the tanks.
And the people in the house had been drinking and washing with that dead rat water.
If you have a tank system, I would suggest you check to see if there is any kind of filtration system on it. If there is, you are the exception.
Sorry to gross you out--but I just want everyone to be informed and check their system.