He's a rabbit: they're scared of everything. Powder browns run the tank. Turn out your light and drape the tank in a black sheet for an hour. That will stop the fight. Then turn the lights on. Every day's a new world if you're a tang...or a duck...or a rabbit. The tang will wake up with the scent of rabbit on the wind and think he's been there for a century, the rabbit ditto---he'll still be afraid of the tang, but he'll have forgotten the fight...
The real danger is that the rabbit is the venomed one, and if pressed hard he could hurt your tang. He's positioned himself where if bitten, the venomed spines will engage and the tang could be painfully stung.