Definitely treat the entire tank using a product called "Jungle Parasite Clear" (JPC). You can get it at most LFS as well as Walmart, and it comes in a tablet form (I think the term they use is "Jungle Tabs"). It contains metronidazole which is the active ingredient, and praziquantal which is another anti-parasite med. Another thing that is imperative (according to the fish veterinarian at my school) is to dissolve a JPC tablet in a small cup of water and soak the fish food in it, and here's why...
The JPC in the tank is unlikely to help the fish eliminate intestinal parasites, but it will kill the flagella that are shed in the feces. (The shed flagella are what makes other fish contract the parasites). In order to combat the parasites within the affected animals' GI tract, they must ingest the metronidazole. So soak your food for at least 5 minutes then feed. You need to feed them the soaked food for at least 5 days-- do not stop before then even if you see the feces improve. The metronidazole does make the food unpalatable so you can add something like Garlic Guard to make the fish more interested in it.
Another thing: before using JPC on the tank, vacuum your gravel really well to pick up any infested poo that is there. Then start with your Jungle Tabs.
Good luck!!!