I have a freshwater aquarium and lately the water has started stink like a rotten egg. I do the 25% water change every other week, condition the water, vacuum the gravel, etc. I don't know what the cause is. The only thing I can guess is that I replaced the 6 year old bio filter with a brand new one about 2 months ago. I didn't put any of that nitrifying bacteria that you're supposed to add when you start a new tank, to help with biological filtration. Could that be it? I added the recommended dose today, hoping that helps. but it REALLY smells bad. The water is clear, and the fish seem happy, but my GF and I aren't happy, the stank is ridiculous. I have had this particular aquarium running for about 10 years now, and tis is the first time I've ever noticed anything like this. Any ideas?