Treating ich when fish don't have signs

I was on vacation and the person who was suppose to be watching my tank did a bad job and as such, I ended up losing a fish to ich. But they put in 3 new fish into that tank and those three new fish are not showing any signs of ich at all. I was told by the LFS that I still needed to treat the tank for ich but they didn't give me a time period of how long to treat it for.



Can anyone give me any advice on this? It's been 3 days since the fish with ich died and the other fish still do not show any signs.