No idea if this can actually be done because it's not currently a feature within MSX however it was suggested I add it here so here it is.

Basically MSX only allows for adding constituents through the [SOURCES] controls, but it doesn't provide any ability for removing them. This would be useful if we wanted to run a high level simulation for something like partial THM removal in a water network.

This can be done by simulating it as a reaction but it's difficult and extremely finnicky. The ability to simply set the concentration to a specific value or lower it by a specific value would be much easier.