That's not what I said.
If fish show symptoms that indicate intestinal infections (not eating well while having a bloated belly, losing weight despite eating well, white stringy poo,...) you treat against intestinal infections.
Ich manifest by white spots on the fish's fins and skin that come and go. The spots are usually salt grain sized and slightly raised but can also be smaller. This depends on the fish's skin, the ich strain and how well the fish's immune system can handle the infection. Usually you will also see the fish scratching themselves on rocks or other hard objects
In such a case you treat against ich.
If the fish clamp their fins, have a whitish plaque on their bodies, display rapid breathing and act lethargic you may be dealing with brook.
... In essence you need to learn to diagnose the infection by the displayed symptoms.