Mixing it in substrate can help buffer alkalinity, but if it is already in filters that may be redundant.
And if you notice alkalinity falling, it may be used up, and more needs to be added.
The pH of my water at the time, was 7.6, and I was trying to raise it to above 8.
To do this, I found adding a few TBPS to the fluidized reactor every couple months was needed.