Poor water quality has nothing to do with parasitic problems although it will lower the fish's immune response allowing the fish to become infested faster. Inverts are unaffected by fish pathogens and bennies/gobies do seem to have higher resistances to these types of problems although no fish is immune for long periods.
How long since the last fish addition?
all the fish and determine if the problem is Cryptocaryon irritans
aka ich which will appear as small sugar sized nodes. In which case the best treatment will be >>hyposalinity
<<. Or possibley Amyloodinium ocellatum
aka marine velvet which will appear smaller like grains of salt or a light dusting. The best treatment there is copper, preferabley chelated or better still Seachem's Cupramine
With the clown you should also keep a look out for Brooklynella hostilis
or clownfish disease. Usually visible by excess sliming on the fish almost like a cloud surrounding the fishs' body. Best treatment would be formalin
In any case of the above, once the fish are removed and QT
'd the main display should remain fallow for a minumum of 4 weeks, preferbley 6.