Still, you must figure out why the alkalinity went from 120 to 300. It's important that you do this.
Can you post some pictures of the aquarium? Maybe we'll be able to see something. Sometimes it can be some old rocks that are breaking down with age.
The treatment methods for ich depend on how serious you need to be about prevention. There is less risk if you have just one small tank and only a few fish. When you have multiple large tanks, and many many fish, then it's a complex situation.
If you have multiple tanks and share the same gravel vac or other equipment for all the tanks, then you'll probably have cross contamination issues. You'll have to treat all the tanks to eradicate the parasites completely. Otherwise the problem could be bouncing back and forth from one tank to the next.
Heat treatment is the easiest method for treating ich. You turn up the heater to about 86F, add more aeration, and reduce the water level in the tank. Heat may eradicate some infestations, but IMO it's not a failsafe plan. If you can visually inspect each fish everyday, then I'd probably just use the heat treatment method. But if you have lots of fish and/or if some like to sleep in hidey holes all day, then you might want to step up the treatment to something more than heat.
There are many products to choose from at the LFS that should work. There are some newer organic products that are made specifically for ich prevention. "Prevent Ich" or "Ich Attack" is what I'd probably use.
http://www.novalek.com/kordon/prevent_ich/index.htm
http://www.novalek.com/kordon/ich_attack/index.htm
Then there is always the salt treatment method. I usually don't recommend the salt method because it takes more work and patience than the other methods.
Here are a few links that will help if salt is the treatment of choice -
http://www.cichlid-forum.com/articles/ich.php
http://www.geocities.com/chefkeithallen ... lator.html
Only thing I disagree with in the article is the water change % after the treatment. I recommend that smaller water changes be done of no greater than 20%.