hmmm i agree that is strange. did the tds go up before or after you removed the plants and added octozin? only things i can really think of is either remoing the planst has dislodged some unseen stuff , or there is something in the octozin that has caused it. does octozin hae the blue/green dye in like sterazin and myxazin? all pure guesses of course , but there has to be a reason for it to suddenly rise.
just thought how you could either prove or eliminate the octozin . test the tds on a sample of tap water , then add some octozin and test it again . if it is higher , then theres your culprit