[Dave Willis]

It tells you you're not chucking enough in to affect the tds of pure water. You are however reducing the surface tension a bit. As the manufacturers won't tell you exactly what's in it then you know no more than me 
If you think it contains nothing but unfiltered tap water then maybe you can explain why it foams so much when mixed in the tank?
I'm happy to be a bonehead 

[SeanK]

It tells you you're not chucking enough in to affect the tds of pure water. You are however reducing the surface tension a bit. As the manufacturers won't tell you exactly what's in it then you know no more than me 

Think about it a little longer Dave... GG4 has a tds reading but there is no tds reading in the water, its not hard mate.