How far away is the 75g? I'd say definitely add more clowns, but other similar shaped species could work too. Keep up the water changes and get the 75g running as soon as possible (but without rushing the cycling process, as someome else mentioned).
One thing worth mentioning from your first post, you said you'll upgrade "before they outgrow the tank". Clowns aren't like Pacu or Arowana, they aren't just going to keep growing to the point where they physically get too big for your 30g (or the 75g, for that matter). IME, you'll want to move them into the 75g ASAP, the sooner the better. Even at a small size they need space to move and grow, they'll make good use of that 4' length even if they're only 2-3" long. And going forward from there, you should also start making plans to move them into something bigger than the 75g by the time they're around 6", IMO. I you want them to keep growing then need space to move.
