In order to completely rid your tank of ich, you will have to remove the fish for 4-6 weeks... I doubt this is an option, and it will require you to QT every fish that you get after for 4-6 weeks in order to stay completely ich free.
Ich on your fish is only part of their life cycle. Fish that are healthy and unstressed rarely get this parasite, and if they do are strong enough to fight it off without too much trouble. The problem is when fish are otherwise stressed, whether because of poor water conditions, bullying by other fish, poor and otherwise unvaried diet, etc. I would think that your hippo died because he was still acclimating to a new environment
and fighting ich... this is all too common, sorry to hear of your loss
