I use this:
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Treats 7,268 litres and cost about $30. Works for me and I used to go through a Lot of water a week.
Doesnt Ollie make and sell his own water conditioner???
As far as the carbon idea goes..
I wouldnt, carbon removes lots of stuff including stuff you actually want in your water.
like the sound of that water conditioner.... but i use prime at about 32-33 bucks for 500ml bottle which treats 5000 gallons... uk gallon is 4.55 litres - us gallon is 3.79 litres - and australian gallon there is 4,22 litres... so a good rule of thumb is 1 gallon is approx 4 litres... so using that conversion 5000 x 4 = 20,000 litres....
i dont like to buy bigger bottles of water conditioner as they can go off... so you think your saving money buying in bulk ( 1 big container ) but then you end up throwing a fair portion of it away....
2 and 1/2 years to use a bottle of water conditioner is too long in my books - definitely go off in that period especially if your careless with cotaminants getting into the bottle...