well i dont see any direct feeding competitors to the mandarin in your fish list, IMO a refugium wouldnt be necessary.
provided your 170 is fairly established, there should be plenty of microfauna for it to feed.
having said that, you could use the 500 still, there is no harm in trying to boost the amount of microfauna in the tank. though it would be a fairly small refugium and probably wouldnt create much of an impact for a tank your size.
if you have a decent amount of rock in your tank thats all you really need. you very well could just add some small pieces of rock and rubble. little critters will grow, thrive and populate in the tiniest of crevices, nooks and crannies.
hope that helps.
