This happened before not too long ago when I had sand substrate. I got it to stop by changing to gravel substrate and I replace the biological filter along with a massive water change, and for a while, there was a bacterial bloom. Now, it smells and I'm not sure what it is because I just did a 50% water change a couple of days ago and it's smelling like rotten eggs again. I have a 29-gallon tank with two goldfish, a pearl scale and a ryukin both about two inches. I also have live plants and an aerator. Any ideas of what it could be and how to get rid of the smell?