You are probably overfeeding the tank if you are indeed seeing planaria. Garlic is good but makes your tank water smell, I wouldn't advise this method to a newbie. If anything just use New Life Spectrum Thera-A pellets.
Have you tested your water? Your tank may not be all the way cycled. 0 ppm of ammonia and nitrite is a must, also for guppies and platies you need water that is on the hard side. Don't give your water to pet stores to test, they almost never say anything useful, instead get your own test kit. Strips are cheap, not as accurate but if you're on a budget 5-in-1 strip kits are $11 at Walmart for 25.
Do some water changes and vacuum the gravel while you treat for ich, it will help cut down on the multiplying pf the ich parasites, which breed in the gravel. I wouldn't use much salt unless you can remove the snail(s) temporarily.
I like this article for ich treatment suggestions:
http://www.cichlid-forum.com/articles/ich.php
Your tank sounds out of whack, you need to figure out what you're doing wrong. Are you overfeeding, not changing enough water? Adding dechlorinator with each water change? Is the filter pumping enough water, does it have enough biological media? These are the questions I want to ask.
Adding some live floating plants such as hornwort or anacharis can help balance things out. They don't need extra light to grow fairly well, if you let them float.