Erythromycin tablets its an antibiotic, you should be able to buy them at most FS or on the web do a search. The dosage I used is 1 tab per 10 gals of water, in 24 to 48 hrs it should all be dead, if not repeat the dose. After each dosage and the 24 to 48 hr time period has elapsed do a water change, before dosing again.
Your killing bacteria and if you have alot in your tank your amonia levels can spike thats why you do the water change.