Fresh water dips are fairly easy but hard on fish . They should last no more than 5 minutes .. you will want to match your PH , temp to what he is in now . RO/DI or distilled would be the best water . But this could potentially cause more stress to the fish opening them up to more diseases . HypOsalinity may be something to try in a QT tank with the lowered salinity the ich cant live in it and die off . Be sure to lower it slowly so as to not shock the fish to much .. I have done both fresh H20 dips and hypOsalinity both had postive effects for me . The fish will usually sink to the bottom during a fresh water dip .. I am not sure if this is effective on black ich or not