This is a simple question. When the upstairs toilet is flushed, the downstairs toilet bubbles up. I realize there is a block somewhere. Can I pour anything into the toilet that may help to clear this up ? I have a septic tank.

You can try septic rid but it really sounds like you need you sewer line cleaned to your tank... did you check to see if your tank was FULL?

You can try septic rid but it really sounds like you need you sewer line cleaned to your tank......did you check to see if your tank was FULL?
I live on the water. The tank is always full of water. I will try your suggestion. It's driving me crazy.
I wanted to be sure it was the pipe from the toilet to the septic tank and not the pipe that goes through the roof before I mess with either. Thanks

The only common way the vent through the roof gets clogged is from animals or leaves... second floor toilet bubbles 95% of time it's the line to the tank...

The only common way the vent thru the roof gets clogged is from animals or leaves....second floor toilet bubbles 95% of time it's the line to the tank....
It's the first floor that bubbles when the second floor is flushed. Do I understand correctly that it is probably the line to the tank? I am getting confused.

Yes... line between house and tank