our tanks are not a 'closed system', ... water goes in (fine, that's not going to have an effect on open or closed)
any tank where water is removed (not by evaporation) is not a closed system.
like in nature, ... if a lake is being supplied water, and there is no output to remove TDS or any other buildup, ... things are going to become toxic to life in that body of water as any and all water that is added to the system is going to bring in additional material.
in our tanks (those of us who try to maintain a more closed system - no water is manually removed) ... the same situation is going to take place if we are not removing anything.
including plant trimmings, ... if we're not trimming our plants and not removing any water, things are going to build up to toxic levels eventually, ... do i have a guess as to how long this could take, ... well lakes in the world that do this are thousands of years old, ... so ... good question on how long it would take in an aquarium we have at home.