Chuck Rush made one out of an under the sink
filter. You can put adapters on the top of it to attach
to a garden hose. The replaceable filter should be
changed once a year. Let me see if I can pull up
his page address from google...
http://www.pondrushes.net/
and the filter is here

Not sure what all it would work on but the unit that
you buy at the store would say what it filters out.

When I add water I use the filter and I let it go out
very slowly and find a rock to run it over (just because
I'm sure, and I have, will forget it is running...) so far,
knock wood, I haven't had an incident with chlorine
poisoning.

Let us know if you build one and how it works!
kathy :-)