In a reality type situation water changes cannot possably maintain original Ca levels or any other level for that matter, unless you are doing 100% water changes.
think about it if you deplete 100 gal of water from 430ppm Ca to 380ppm Ca and you do a 20 gal water change with 430ppm CA is it going to raise the Ca level to ~390ppm, a 50% waterchange would only raise it to 405ppm. so if your tank consumes 50ppm in 2 weeks and you do a 20% change ever two weeks (which is a lot more than most do) after the first two weeks you will be at 390ppm, afdter 1 month you would be at 358ppm, 6 weeks 332ppm and 2 months 312ppm. please don't bother with the math this is aproximate as I didn't want to work it out this lait befor I head to bed, but it does show you what will happen on a tank with a high Ca demand.
as you can see you will need Ca addition other than from water changes.
Alk will be afected in a simular mannor also.
now for a lightly stocked tank this could go slower but you are
never never never and incase you didn't get that I will say it again
NEVER 
going to maintain your levels with just water changes unless they are 100% changes.
Steve
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