Just a word of warning guys as all of a sudden I am seeing different brands of mixed bed resin being sold. Do not take it as being fit for water fed pole as the product says mixed bed resin.
There are various forms of mixed mixed resins on the market, one type that will produce pure water and other types that will not and some will produce sulphuric acid.
The key factor in mixed bed resins is what types of cations and anions that are used in the product. The Cation needs to be in Sodium Hydrogen form . If the Cation is of Sodium form then you will have problems. Same goes with the Anion resin.
Don't take for granted mixed bed means water purification, some don't.
A good site is Reskem.com which is full of data. There is a product on Ebay on the moment and having looked at the spec is not water purification. Just be careful what you buy.
Cheers
Doug