In my experience, multiple water changes, (large or small) in a new tank that has an amonia issue is a bad idea. I had a very simmilar issue a couple years back with a tank I had just set up recently. Everything was perfect for about a month, then bam! Huge amonia problem. I thought I needed to change the water more, it seemed to get even worse. I changed the filter because it seemed fairly dirty. I definatley got worse. I lost 3 fish, and was about to lose my mind. My local pet store owner told me to use amonia lock 2 (I believe that was it) and use ocean salt. Even in fresh water tanks it is good for your fish is small amounts. I put in the amount it said to for helping heal fish (1tbsp/ 10 gallons) and used the amonia lock. Within 2 days all was well, and using the salts everytime I change the water has kept the amonia gone. I used to change the filter media every time I changed the water. DON'T. No matter how dirty your filter media is, give it at least a day to help the natural bacteria build back up. Without the bacteria in the water, nothing is able to fight against amonia.