Hey Mitch, if you want, look up something called a Hydor Mini Heater. It's rated up to 10G, and it looks like a small pad and would definitely fit in your AC50. Not sure if they sell it in Australia, but it's worth checking out. Very cheap too. Cheaper than most heaters you can buy. And reliable from what i've seen. I used one for my 10G FW once. Never had a problem with it.
Just giving you ideas..