I keep several containers with rainwater for various cp's as the local water here is very hard...it filters through a limestone aquifer. One of the containers I use is a 33 gallon water softener salt exchange thingie with the guts stripped out and cleaned of salt. It is getting a bit long in tooth but still does its job. Recently lots of mosquito larvae have gotten into it so I threw a few sprigs of Aldrovanda in it. It does not get much light but the plants are doing quite well.