I cleaned and added new Fernox inhibitor to my C/H 18 months ago, I have now replaced an old rad in the Bathroom and the new one is plumbed in and waiting to be filled, would it be advisable to add more fernox into the header tank or just fill the rad with plane water. If I add fernox shall I tip the whole contents or part? .....my main concern is, will too much fernox harm the system ?