The golf balls are hollow, so sure it's not as much available space as bio balls, but they were a lot cheaper than the bio balls, and you can get them anywhere. But in saying that I found some cheap bio balls, so have included them in the mix.
The system itself, is pretty much home made job, freshwater leaves six tanks into a piping system which falls into a tank. First up in the catchment tank is the basket filled with cotton wall. This leaks into the second section filled with bio ball/golf ball. Under near that the third section filled with gavel and an air stone pumping out air to circuit the gavel and bio/golf balls. Four section is a course sponge, followed by the last section a normal sponge, before the water is returned back into the six tanks via a pump.
This is version two, and the only problem I have had (so far) was a bristlenose swimming into the pipe ending up in section one. She was only discovered after investigating the smell.
Should the pump fail the catchment tanks holds the access water (just), should that fail the hole things sits in a large container, so no water on the floor.