So - I'm worried that it could be flukes, and a FW dip would help you diagnose that. I agree, they are a bit tricky to do and hepatus tangs will just about look like they are dying when you dip them, but if it has Neobenedenia (eye flukes) the dip will tell you that. If you do decide to try a FW dip, there is some info about it at the top of this section, and here is an excerpt from my upcoming book about it:
Osmotic differential baths
Formalin is not the only irritant that can be used to dislodge parasites during a diagnostic bath. Osmotic differential baths (exposing a marine fish to freshwater, and vice versa) are also effective at loosening parasites. However, there is a greater possibility that the shape of the organism will be disrupted by osmotic pressure to such an extent that proper identification would be difficult. “Freshwater dips” are commonly used to dislodge parasites from the skin of marine fishes. To perform these dips, the fish is captured and placed in a dimly lit container of clean freshwater the same temperature as the aquarium system it came from. The dip container should be covered with a clear material so that the fish is kept from jumping out, yet can still be observed for overt signs of stress. Hold the fish in the freshwater for five minutes to seven minutes. If acute stress is seen, the dip may need to be terminated early. The idea is to dislodge the parasites, without unduly harming the fish. Many aquarists opt to use reverse osmosis water, and adjust the pH to match the aquarium, but this is simply not necessary if the pH of the freshwater is between 7.5 and 8.5. Some advocate to not aerate the water during the dip, but the water motion from air bubbles can help keep the fish active so that they can be better assessed for stress. Additionally, the air bubbles help dislodge some parasites during the dip process.
Some caveats:
- Some fish will give extreme reactions to being dipped (hepatus tangs for example), as long as they were swimming well before the dip, it is best to ignore that behavior and continue for a full five minutes.
- Moribund fish can be dipped, but understand that the acute stress from the dip may prove fatal. The dip water should still be checked for parasites in order to possibly help any remaining fish. Moribund would include any fish that was easy to capture or a fish with a respiration rate of 200+ BPM.
- Choose the type of net wisely, some fish have spines that will become caught in the netting. Have a pair of scissors handy to cut the net if this happens.
- Dropping a fish while moving it to and from the dip is common, take care. Some fish have venomous spines, so know that before trying to handle any fish.
- Fish shaking their heads violently after capture is one cause of mechanical exophthalmos (popeye), so try to gently restrain them in the net while moving them.
