It was at 001PPM coming through the vessel on Wednesday, 002PPM on Thursday. About 500 litres have passed through during this time and I shake the vessel before every tank fill. I've just checked the TDS and it has shot up to 014PPM . The same thing happened last time the resin had gone off. I use Tulsion MB-115 by the way. Now, I have a small amount of resin left (about 7 litres) and a brand new bag. Would you be tempted to use the 7 litres from the old bag and top up with the new bag or fill the vessel with the resin from the new bag? I'm tempted to use just resin from the new bag and keep a close eye on the TDS when it starts creeping up. If it does shoot up like my last lot I think I might try another brand for a change. I bought the latest bag from doug atkinson, so I'm interested to see what my findings will be. Anyone else noticing a quick rise in TDS?