food that is grown the way God intended (ie organically without poisons) is good for you, in balance. everything else is suspect. eating two bananas a day is good, eating 10 a day is not so good, just be balanced. margarine is full of trans-fat and should be avoided, as should white flour, White Sugar , and all highly processed starchy sugary foods.
cherries? sounds great, i say eat em up. you can't go wrong eating fresh organic (when availalbe) fruits and veggies, and don't listen to fear mongers. the person who told you about bananas and cherries probably went to eat at mcdonalds after that. what does she know?
now, heavy metal in fish IS unfortuanately something ot worry about. we have poisoned our oceans so you have to be careful. you can buy fish oil that is mercury free. but just keep the fish to a minimum, no more than 2 or 3 servings a week, i know of no other method to deal with mercury in fish.
you may want to get a good book on a healthy diet. there are so many like "the makers diet" or the rainbow foods diet (forget the actual name). maybe look at the book recomendations in the "raw food" forum.
growing your own sprouts is a WONDERFUL addition to all diets. check out the sprout forum for great info on that. hope this helps a bit.