You will have to stop doing water changes while you treat but if you get ammonia or nitrite readings then do a big water change and retreat.
Just follow directions on pack, should be 1 tablet for 40 litres (10 gallons).
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 form the top of the substrate to the top of the water.
Use the calculator / converter in the "How To Tips" at the top of the page to convert litres to gallons.
Triple sulfa is a pretty safe medication and can be used on all fish and inverts so is quite safe. Treat the tank for a week and see if it helps. If it does then stop treatment after 2 weeks and do big water changes to dilute it out.