Hi and welcome to the forum
Can you post pictures of the sick fish so we can confirm white spot?
What is the ammonia, nitrite, nitrate and pH of the aquarium water?
You might have overdosed the medication. The best thing to do is a 90% water change and gravel clean the substrate.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If it is white spot, then raise the water temperature to 30C (86F) and keep it there for 2 weeks, or at least 1 week after all the white dots have gone. The heat will kill the parasites without you needing to add medications.
Don't bother with salt, it does nothing to white spot. The heat is safer than medications and all tropical fishes can tolerate it for a couple of weeks.
Increase aeration when using chemicals, salt or raising the temperature because these things all reduce the oxygen level in the water.
The following link has information about white spot, including various ways to treat it. Post #1 and #16 are worth a read.
This is a common question that is often asked, what is ich and how is it recognisable and what causes it? The real term is ICHTHYOPHTHIRIASIS. OR commonly known as white spot. It is an extremely comon parasite that affects aquarium fish. It is highly infectious and potentially lethal and...
www.fishforums.net
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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.
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.
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.