Flashing, when a fish rubs himself on the gravel, decorations or plants, is normal. Many scientists and master fishkeepers cannot answer the question as to why fish display this behavior.
Ich is a parasite. Part of its life cycle is to attach itself to a fish. I do believe that it is always present in a tank. It does have very specific needs. Temperature is one. By maintaining your tank at the upper temperature levels required by your fish, you prevent the ich from having the needed element to hatch and attach to your fish. I have found this to be very effective.Keeping your fish healthy is also key.
Do not use your medication arbitrarily. Some fish cannot tolerate the medication full strength(example would be some tetras). Some tankmates must be removed when dosing(some inverts). Plants may also suffer when using the medicine. Most importantly, follow the directions precisely.
The bald spot on your severum sounds like missing scales. This could be from attacks by the blood parrot, a very aggessive fish. BP's are not a great community fish.