We spent a lot of time on Ich in my aquatic medicine and fish immunology courses in college.
Ich is a ciliated protozoan that is not killed by domestic water treatments so if the water source has Ich, then your tap water has Ich. Unless you used Ich meds on every water change you ever made, then there is a good chance your water changes have larval Ich in them.
Medications only work on the free swimming stages of the protists life cycle. Thats why you increase temps--speeds up the life cycle and increases your chances of catching the swimming stage with the meds. Ich meds are just supposed to kill off the critters long enough for the infected fish to recover and not be reinfected in its weakened state.
The reason all of our fish don't get Ich with every water change is because a fish's immune system is plenty to protect them from Ich if they are healthy. When fish get stressed from environmental conditions or their fins or slime coat is damaged, then they become susceptible.
Thats why I love NovAqua. Prevention is literally the best medicine. Keep an eye on your water params, handle your fish very carefully, watch out for fish on fish violence, and feed a high quality food. Nutritional deficiency is a huge stressor on a fish.
If only one fish has the Ich then something probably happened to it or its just getting old. Just keep it quarantined and treat it alone. Just add a teaspoon of salt per gallon to the main tank and maybe something to help out your fishes' slime coats. A teaspoon of salt per gallon shouldn't be enough to hurt the plants. You can get in-tank UV sterilizers for pretty cheap now adays so they are a practical addition to quarantine tanks and they do kill Ich swimmers.
I've manually cured a light Ich infection in the lab by doping the fish with MS-222 and then removing the cysts one by one with forceps. NOTE: I had a fish surgical station, don't try it in a tupperware container at home. Then I plopped the fish in a quarantine tank, 1 Tablespoon of salt per gallon, high filtration and aeration, kept the tank dark, and it recovered just fine. Golden shiners are probably a little more resilient than say CPDs though.
This site has a pretty good crash course in Ich.