Donal. When Thetford entered the toilet market (it used to be only Racasan and Elsan) their 'pink' top fluid always left a clean flush and their 'blue' left a clean bottom tank. Now, their formulations are different and you can buy additives (from Thetford) one of which will stop the top tank getting spotty and a further additive will stop the bottom tank scaling up.
Me, I've gone back to Elsan which has top and bottom tank fluids which are complete. I also find them cheaper - remember Thetford may have a virtual monopoly on installed toilets, but you can choose your consumables.
Peter T