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?
Quarantine 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.
Cheers
Steve