plants probably wont do much in the way of hardening or softening your water.
baking soda added to the bucket at water changes before it goes into the tank works wonders for kh and ph (gh too? not sure), however it is a very touchy thing and there is a very specific ratio to add to adjust it 1 degree etc (which has escaped me).
i read the oxy shell things once, its just bicarbonate of soda or something like that in a fancy block but it will work.
if you have a filter in your tank with places for sponges etc buy yourself some bird grit (the stuff birds eat). stuff that into a stocking, wash it and pop it in your filter. it works wonders to get everything up but you do need a fair amount, ie for my 220L i used a whole pack (350g) in my external and it got the kh and gh up from 2 to 5, and the ph from 6.8 up to 7.4. alternatively you can pop a few nice clean shells around the tank (sand off sharp edges).
if you can get a base reading of your tap water it will help alot, there may be something in your tank adjusting your parameters that you havent found yet.
hope this helps