Amazing tap water filtration results!

So I have one of those Pur water filters that screws onto your sink faucet. Decided to test the TDS level of plain faucet water and then Pur filtered water. There was NO difference. My tap PPM is 50~ppm. So I have really good tap water and the PH is 7.1 always. I picked up one of those $17 Brita water filters-the kind that you pour the water in the top and it filters down into a container. Well, I put a cup of water in the thing and tested the output water and amazingly the PPM had dropped down to 30ppm!!! I then re-filtered THAT water and it went down to 23ppm. After the fourth filtration the water PPM was 11ppm!!! I then took a cup of water and put a bit of nutrient solution and salt into it and tested the PPM - it was 500ppm. I filtered it once and it went down to 300ppm. By the fourth filtration it was down to about 80ppm. Now I realize that those things aren't practical for filtering water for a reservoir-however, if you were to spend $40 and get 3 of them, you could easily daisy chain then with a reservoir underneath and slowly drip your tap water and make a whole reservoir full. Assuming your water is 300ppm out of the tap, you could create 1 GPM of 100ppm water out of this system. Food for thought. I was able to make 10 gallons of water in about 15 minutes just using the one filter that I had.