Sorry to hear that. You have some work to do as well. I had this problem happen to me before(others putting fish in my tanks). Best bet from my experience is QT the fish and use hypo salinity treatment. Please note Marine Ick is a pesky parasite, it can stay in a fish for up to six months so please use a reasonable size tub or tank, you my need lights can use cheapo ones since it is for the fish even regular PC bulbs. Now for the main tank. If your tank is reef you will have to look in to how low salinity levels your corals can handle and drop as needed will need ta try to starve the parasites. You will need to do more research. I have used Kordan Ick Attack to treat the parasites in the tank while I had my fish in QT, It worked pretty well note I have not tested on a reef tank so again please do your research. Also a Diatom filter can help remove the free floating organic compounds and bacteria from the water.
Best Regards,
V......