When water sits in a hose, chemicals leach out of the plastic into the water, and it will foam up like you are seeing. Most hoses are labeled with a warning not to drink from them. I let the water run until all the bad stinky stuff is gone (I can smell it), then I fill the waterers. It shouldn't foam then.
Rabbit poop doesn't have to be composted (rotted/broken down) and can be put straight into the soil around plants without harming them. Most other manure is considered "hot" and will burn plants/roots if it's not composted first.
