I'm new to the ro. Had my tank filled up so went to turn the water on and tested the water and it was coming out at 6. I've a 450gpd Collins. I flushed it for about a minute and it went down to 1 then 0 and then closed over the flush and it seems to be staying at 0. Is it procedure to flush them every now and then? And when the tds creeps up how do you diagnose wether it's the resin needing changed or the pre filters?