The first thing you should do is test your water with Tetra EasyStrips, for high ammonia and nitrite levels. A bad smell is usually a sign that excess waste is in the tank, and usually excess food. To correct the problem, change out 50% of the water, wait one whole day, and then change out 25% more of the water. Make sure when you feed the fish that they are eating all the food in two minutes or less. You should have one gallon of water per one inch of tropical fish. Have three gallons of water per one inch of goldfish. Make sure the water is properly filtered as well. Ne sure to use Tetra AquaSafe® when replacing water to make your tap water safe for your fish.