If it was ich, then it means its still there. Your fish could continue to fight it off... or not. No way of knowing until the ich shows its head again. Even if the fish can fight it off now, when you add another fish, the stress level in the tank will go up for a while until everyone gets used to each other. When things get stressed, the immune system usually doesn't work as well as usual - and they're wide open for the ich to take hold again.
The only way to really feel safe the tank is clear is remove the fish from the tank and treat them for ich. While that's going on, leave the main tank fishless for 8 weeks to break the life cycle of the ich and kill it off.
Here's a good article: Marine ICK - everything you need to know