If there are any fish in a tank with ich then the entire tank is infected. To safely treat the tank without using meds raise the temperature of the tank by 1 - 2 degrees per hour until it hits 86 degrees then keep the temperature this high for approximately 10 days or 3 days after there is no longer any sign of ich on any of the fish. This is by far the most widely accepted and safe treatment out there for fish and can be used on just about any tropical fish.
If you choose this route just be sure to increase aeration by either dropping the water level by a few inches allowing a HOB filter to splash more or you can use an airstone. Higher temperature in the water holds less oxygen so it is better to increase aeration rather than risk a death from suffocation.
Take a look at this link for some reading on the topic.
Understanding and Treating Ich or White Spot