Trapping the fish would stress them out, I would feed heavily, keep stress down by keeping out of the tank a healthy fish should fight off the Ich. then trap and Qt.
Trapping them is the least stressful way to get them out - and the hippo got to go as it will be under constant stress in a small tank.
Also heavily feeding will do little to nothing against ich, but rather the opposite.
By the time you have an escalating ich outbreak, intervention is needed. Hoping for the fish to manage it themselves at that stage will likely lead to losses.
If the infection level goes down after the hippo is out no further treatment may be needed, but the parasite will remain in the system.
If you want to get the parasite completely out, you would need to take all fish out and leave the tank for a couple of months (minimum 3) without fish.
At the same time you need to clean the fish themselves from the parasite via TTM, hyposalinity,... whatever works best for you and the fish.
You will also need a quarantine tank and a rigorous quarantine protocol for everything new going into your tank.
BTW. In a tank of that size tangs are a no-go or you will always have issues.