We used r/o water to fill a leaking outside fish pond when the owner did a 'duck'.
The sediment filter will remove larger grit, the chlorine filter will remove chlorine (which is harmful to fish) and the r/o will take anything else out.
When we used to fill our inside fish tank after a clean out, we filled the containers with tap water and left them to stand all day outside. The sun removes chlorine from the water, which is why we always chlorinated our swimming pool on an evening and not in the morning.
I know a chap who just uses one 10" filter through a carbon block/GAC filter to fill his fish pond.