How do I get the pH down, naturally?

#1
I'd like to get an angelfish for my 36 gallon tank, but our water here is very hard (pH of about 8.4). I've been reading that they like softer water than what I have. Without using chemicals, how do I get the pH down? I have a Whisper HOB filter for that tank, is there something that I can put in the filter to help? I think I read about something, but I can't remember what it is.

Any help is always appreciated!