I've been using EC for a few years now and never seen a rise in my parameters. Tap water is a 7.6 and tank water is a 7.2. Could there be any calcium content in your hardscape that the EC absorbed?
thats what i was going to ask. there are hundreds of other possible causes for the rise. myself personally i have never had a problem. i've had it running in 3 of my tanks (29g,20l, 10g) and never saw any changes.