Have you checked the water quality for ammonia and nitrite?
Can you post some pictures of the fish?
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Giving marine fish a bath in freshwater does nothing to white spot parasites.
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You need to be very careful using chemicals in marine tanks because the rocks displace water and you can easily overdose and kill everything.
Before you treat the tank, do the following things.
Work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
If you have big rocks or driftwood in the tank, remove these so you get a more accurate water volume.
When you measure the height, measure from the top of the substrate to the top of the water level.
You can use a permanent marker to draw a line on the tank at the water level and put down how many litres are in the tank at that level.
There is a calculator/ converter in the "FishForum.net Calculator" under "Useful Links" at the bottom 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 adsorb the medication and stop it working.
Wipe the inside of the glass down with a clean fish sponge.
Do a 75-80% water change and gravel clean the substrate. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn.
Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water.
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The safest treatment for white spot in fish tanks is heat. Raise the water temperature to 30C (86F) and keep it there for 2 weeks. The parasites can't tolerate the heat and die. No chemicals needed.
Before you raise the temperature, do a huge water change and gravel clean the substrate, then clean the filter in a bucket of tank water and re-use the media.
Increase the aeration/ surface turbulence too because the warmer water holds less oxygen than cooler water. You should do this when treating with chemicals too because they also reduce the oxygen levels.
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The other safe way to treat white spot in marine tanks is to move the fish to a clean container of salt water every day for a week. The parasites drop off the fish and when you move the fish, you leave the parasites behind. Each day after you move the fish into a clean container, you tip the old water out and wash the container out, then let it dry until the following day when you set it up with clean water and move the fish into it.
This method requires a lot of salt water and is time consuming but works and you don't need chemicals.