I'm not sure if these are legal in Australia or not (from what I've heard, Australia is pretty strict on non native aquatic creatures and has made quite a few illegal), but I think it'd be a good idea to try selling some Malaysian trumpet snails as tankmates for your sea dragons...
I think they would be good as:
1) They can tolerate anything from pure freshwater to pure saltwater.
2) They are fast breeders.
3) They have a different function from the winkles and coco snails as they will not eat much algae, but they will specialise in eating detritus.
4) They burrow in the sand, stopping the sand from becoming anaerobic, and as you probably know, anaerobic sand can kill your creatures.
