keep an eye on the remaining fish for signs of internal parasites- they include being bloated or skinny, white, mucousy poo, going off their food (or even sucking in/spitting out food) flicking or rubbing. if the fish have mucousy, stringy clearish or shite poo time to treat (usually the most obvious sign). Im in US so im guessing our meds are generally the same. Find a good med which contains Metronidazole, and praziquentil. Camallanus worms can be a nightmare to get rid of. if any of the other fish have red threadlike (almost like a pom pom) comming from their bums, another sure sign its camallanus. I honestly would treat anyway just to be sure. doesnt hurt to worm the fish.
Just checked my fishy medicine chest- Jungle brand makes both a medicated food and a tank treatment. May be worth checking the fish shops to see if they carry it.
all the best!
cheers