FE,

Deworming the fish is when you treat your fish with medication that is supposed to kill parasite worms living inside your fish and/or fish tanks.

The frequency that you deworm your fish will depend on how often you feed it with food that's not commercially prepared, e.g. treat more if you feed your fish with natural foods compared to feeding with beefheart and treat more if you feed your fish with beefheart compared to feeding with commercially labeled flakes, bits, etc. It's a good practice to treat fish that you have recently bought, i.e. during the quarantine period.

I use praziquantel for deworming. The commercially prepared Parasite Clear is also good.

I have treated fish that looked "normal" but still had worms. Go to this link to see the pics http://www.myfishforum.com/threadnav2869-2-10.html