Science/Chemistry question:

"KurtG" wrote in message ...
btw, it also drive me nuts when the recommendation is to "replace your
evaporated water first" then do a water change. You want to suck out
waste water at it's highest concentrations and then refill will both
salt and fresh water to get you back to your SG. Of course, that's why
I use two trash cans rather then one.

Recomendations are just that: recomendations... :-)
You do not have to follow recomendations if you KNOW WHAT YOU ARE DOING :-)
And in this case you are of course right. I do a water change first,
I use empty Instant Ocean salt buckets. I prepare two full of sal****er
and use third for drain. If you replace same amount of concentrated
water with diluted one your salinity will drop, so you need to drain
little less old water to replace it with the new...

It takes some practice, so for beginners recomendation is good: after
replacing evaporated water and bringing salinity to normal you simply
replace same amounts of old water with new water and salinity stays unchanged.

But, to each their own... You can always use larger or more frequent
water changes to accomplish the same level of exchange, but it seems
like a waste of salt and effort to me.

Exactly - larger water changes are simply much more effective.
And this is not linear equation: replacing twice 10% of water
does not equal one 20% water change. 20% is much more effective.