how often should put root kill in my septic tank?

Answer 1

The more important question/answer might be, Why the need?

To eliminate the intrusion, you might research what "roots" are intruding into the plumbing from the house, as well as what surrounds the Septic tank.

You don't actually state a problem. Is there one?

With no offense, I'd have to wonder about any installer placing a septic in proximety to trees, for one thing.

If there is only suspicion? Do some investigation. If there are drains that run slowly, or worst case scenario, "roots" invading the bowl of a toilet, then I'd say "YES" you have some issues. If this is just some form of mind soothing, pre disaster remedy, I'd consider the "Why" of it, and resolve that issue, hence freeing the ground from the toxicity of chemicals.

Steven Wolf
(The Rev.)

Answer 2

monthly(get the tank replaced asap) but monthly treatments of root killer should be enough ....only one way to tell try it you may have to increase the time interval