Plants won't mind it too much unless your Ph is already very high. I've used limestone before (quite a large amount) to hold some wood down for 6 weeks. The tank had no fish in it so can't comment on any effect on fish.
Whilst it will raise the Ph there is a lot in the tank trying to lower it i.e. wood and CO2.
So whilst the CO2 and wood and other things are lowering the Ph, this increased acidity will errode the limestone quicker.
If you have fish then I don't know what effect it will have on them but it won't affect plants. at least not in the short term.
Why did I choose limestone? because it was what I had to hand with living in a limestone area.
There is about 5 kg of limestone here in 125ltrs of water. You can't see it all but there are 8 large pieces holding the wod down. It came out once the wood was fully soaked:
AC