There is a great article at aquahobby Using Heat to Treat Ich in Freshwater Tropical Fish - Article at The Age of Aquariums - Tropical Fish
that helped me back when I was having trouble with ich.
I used a heat treatment only. Some fish species will tolerate this better than others. ( my cherry barbs all died, but lemon tetras all lived) so be aware that some can not tolerate the temps.
Basically you slowly bring the temp up to 87 F degrees (32 C) some will bring it up even higher and then you keep it there for 10 days. You may experience some deaths but eventually you will eradicate the ich.
It is important to note that you didn't do anything wrong. It is possible to get ich even when you properly quarantine all new arrivals and everything looks fine.
Keep doing those water changes too.
Your beneficial bacteria should be able to survive the heat treatment just bring the temp up slowly over a few hours time.
Best of luck to you. I know it is scary but it happens to just about everyone at some point. You will beat it and your tank will go on to be healthy.