FishNoob wrote in
:

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

It depends on the grade of the limestone and I imagine on the softness of
your water too.

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?

More or less. It depends on how fast it is going to buffer up your water.
A KH test kit will let you keep track of that. You can always put it back
into your system after a water change. If you are going to be doing huge
water changes regularly, than it may be best to just leave it in all the
time. Test and make a judgement for yourself.