All medications can kill fish if they are overdosed.
A single big water change will dilute things more effectively than two smaller ones.
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 are 3.785 litres in a US Gallon
There are 4.5 litres in a UK gallon
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.