i have a 20 gallon tank and would desperately appreciate some help. i have only an oscar left in it as all of the fish keep dying. the water keeps turning green. i have changed the water and cleaned everything twice and within 3 days it is green again. i did a quick dip test strip on it and it says water is 300 ppm for hard water. will this turn the tank green and what can i do to correct my problem?