If I read your post correctly, you test
after a water change
I prefer, and recommend, you test your paramaters
before a WC
Personally, I like to know what's been used and other stuff
before a WC so I know what is going on and what I may need to do to keep the Big 3 stable
Earlier in my venture, I also tested a day after a WC to see what was up
Testing after a WC doesn't give you enough info as it's just what you've changed now, not what's been going on in between the changes
Maybe others have different opinions, but, when I'm adding New Salt Water with readings of 10-11 Alk, 450 Ca and 1400-1450 Mag to my system, it makes a difference to me how much and how often I do WCs
Changing 15% bi-weekly is very different for me than when I do 10% weekly
This is because of my NSW content
Hope this makes sense, I'm tired and off to bed
