If the fish has white spot, you need to treat its tank because the disease is in its tank.
Are you sure the fish has white spot?
Bettas are normally kept on their own and this reduces the chance of them getting white spot.
If you overdose with any medication, you can kill the fish or invertebrates (shrimp, snails).
You can treat white spot with heat. Just raise the water temperature to 30C (86F) and keep it there for 2 weeks, or at least 1 week after all the spots have gone. No chemicals needed and shrimp safe.
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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.
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.
When you measure the height, measure from the top of the substrate to the top of the water level.
If you have big rocks or driftwood in the tank, remove these before measuring the height of the water level so you get a more accurate water volume.
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.
Remove carbon from the filter before treating with chemicals or it will adsorb the medication and stop it working. You do not need to remove the carbon if you use salt.
Increase aeration/ surface turbulence whenever you use heat or chemicals in the tank because they reduce the oxygen in the water.