Hi and welcome to the forum
Pictures of the fish?
You probably overdosed the tank. Most tanks sold as x gallons don't actually hold that much water when they have gravel and ornaments in. They usually hold less.
The safest treatment for white spot is heat. Just raise the water temperature to 30C (86F) and keep it there for 2 weeks, or at least 1 week after all the white spots have gone.
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Before you raise the temperature, do the following:
Wipe the inside of the glass down with a clean fish sponge.
Do a 80-90% water change and gravel clean the substrate. This will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
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. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration to maximise the oxygen in the water.
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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.
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.
