How to treat internal prasites?

[wonword]

So, im not sure what it is, but something keeps killing off my fish in groups. The first one dead was a perfectly healthy big Leiarius pictus catfish. Because he didnt appear sick, I thought maybe he just had a heart attack or something. A couple weeks later, my two motoro rays died the same day. Now a couple weeks later, my 3 large corydora brochi died last night and this morning. Now I just want to save my datnoids, can anybody help me out?

 

[Trikle123]

Ive used this stuff in the past, works good.
http://www.fosterandsmithaquatics.com/product/prod_display.cfm?c=3578+4615+4620+4756&pcatid=4756