Furan 2 is an anti-biotic and does nothing to whitespot (assuming that is the problem).
Posting a picture of the fish might aid in identifying the problem or confirm whitespot.
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Black ghost and elephant nose are easy to keep but sensitive to all chemicals because they are classed as scaleless fish. If you ever have to treat scaleless fishes, you should use a medication that is suitable for scaleless fish like eels, loaches and catfish. If there are no instructions for use on scaleless fish, use at half strength.
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To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will absorb the medication and stop it working.
Wipe the inside of the glass down, do a 75% water change and complete gravel clean. And clean the filter before treating. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.
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If fish ever react badly to medication, do a 75% water change immediately to dilute whatever was added to the tank.
There is more information about what to do if your fish gets sick, at the following link. It is long and boring but worth reading. I recommend printing it out and reading it in bed to help fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/