From my experience fish dont seem to be too concerned about the water I put in the tank, but it is a 75gal.
On my shrimp tank, it is a different story, it is a 20gal tank, and I was having major deaths regularly.
I am using a Brita pitcher filter because it also eliminates copper.
I only change roughly 2 gallons of water every week.
I figured that each filter will last for 40weeks, which is very reasonable to me.
The water coming out of the filter shows a ph of 6.4, gh 40 and kh40.
Using epsom salts I increase the gh to 100 and using baking soda I increase the ph to 7.2, but unfortunately KH goes sky high to 240 (which is basically what comes out of my tap water, only difference being that ph is @ 7.8.
I dont know how to get to a ph of 7.2 and have kh down to a more reasonable level (100-150)?
So basically I am replacing 2gallons of water every week, and already I've noticed less deaths.
And because the water is being repalced gradually it doesnt affect the shrimp.
My hope is that the shrimp will strive better in a lower ph, lower gh and somewhat high kh.
Oh yeah and no copper too.
(That was the goal of getting the brita filter).