I have used "MELACH LIMON" in order to get rid of limescale (on hebrew "AVNIT") in the hot water tank for shabbat. I have realised, that this MELAH LIMON has a hechscher, but it is not kosher for passover. Is it chamez or can I use this water tank even I have used a product which is not kosher for passover? Thanks for your answer indeed and best regards

Most Ashkenazi poskim do not allow citric acid for pesach, but having used it to clean the tank does not prohibit the tank for Pesach since the citric acid was not fit for consumption in the state it was used.