In article ,
says...
It's for raising the pH?

Yes, that's right.

It sounds like crushed limestone or something
similar. It will certainly raise your pH by buffering up your carbonate
hardness.

You should get yourself a KH test kit so you can monitor how much this
"LITH" is buffering up your water. For a planted tank you probably don't
want your KH to get much higher than 4 degrees or so. If you don't monitor
it the limestone will continue to leech into your water regardless of how
hard you want your water to be.

I'll look for a kit. How long is it likely to take for the limestone
to make a difference?

WRT it keeping leaching into the water - doesn't it need to continue
to do that to compensate for the softer water introduced at water
changes?

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FishNoob