How long is it taking for your KH to drop? And, why do you suspect the plants are doing it? In my case, I had both KH and GH dropping to 0 over the course of just a few weeks. I eventually tracked it down to an acid source in my tank. I had some driftwood that was leaching some form of strong acid(s), which were combining with the carbonates and such and 'eating' both my KH and GH.
I'd suggest taking samples of stuff from your tank (e.g. a little substrate, other decor pieces), placing them into tupperware containers along with a control sample of just water and then recording any KH/GH changes over the next week or so.
My suspicion is that something is leaching an acid...unless of course this is happening over quite a long time, in which case, it could be the plants.
Just something to consider,
46g bf, XP3 w/ vertical spray bar, CO2 w/ SMS122 & diffuser,100% Flourite, 96w+55w cf AHS, AGA Member