Sorry to hear about the other Clownfish, that's a rough way to start out in the hobby, but unfortunately all too common.
I'm not so sure if this is Ich, I think erratic swimming points more towards something like velvet, but it does sound like you have a fish disease organism in your tank, and the typical method that's often advised is to allow your tank to go "fallow" or "without fish" for an extended period of time. I believe Jay H. recommends as short as 45 days for Ich with an elevated tank temp (81F), but I think he advises a full 60 days for Velvet. You could do 60 days just to be safe. In that time any fish parasites will die off without a fish to complete their lifecycle.
Then going forward, and as mentioned above, you may really want to look into either purchasing pre-quarantined fish from a reputable vendor, or QT'ing your own fish before adding them to your display tank. Here's the correct R2R QT protocol that I like to follow:
https://www.reef2reef.com/threads/current-quarantine-protocol.825055/
