I daven in a minyan where some people have instituted a new minhag - to say Tehillim chapter 130 after each shachrit. I object for a couple of reasons: (1) Saying the same chapter of tehillim every day can be considered to be an addition to the davening - something we do no thave the power to do. (2) Tehilim 130 was instituted by (?) to be said during the 10 days of repentance, not every day. (3) By saying tehillim every day we are making a mockery of the very old concept of why and when we add tehillim to the end of davening. (As an aside - one member had a sick relative, so the gabbai announced that after we say the "Shir Hammalos" (130) we will say a chapter of Tehillim for the sick member’s relatIve!) Am I off base?
Tehilim ch 130 is commonly said in times of trouble or preceeding a prayer for a sick person. It is certainly possible for an individual or congregation to say this chapter after davening. The question though is: 1. does this become a Minhag which has status and cannot be undone? 2. Can an individual leave the synagogue before the congregation has completed the chapter? It would seem that to make this the Minhag of the shul would require a decision of the Rabbi, the Gabbaim, and possible a majority of the congregation. If some people just started saying it, it does not become the "Minhag of the shul". On the other hand, if the custom was introduced by communal approval, for whatever reason, to retract the Minhag would be problematic. (I know no reason why a congrgation cannot add chapter(s) of Tehillim to the daily routine, and many congregations do so.) As to leaving, I believe this is not a problem, as the people who set up such customs understand that there may always be people who want to leave the synagogue for a variety of reason, and there is no reason to believe that a new stringency is being intrduced.