HI all, I am new to this website but I have came in hopes of finding someone who is able to give me answers as to why my tank-which has been up and running for 4 years-is falling apart. Its 55 gallons, with two filters, two air stones, a heater, and around 7-8 thriving live plants. I have had a variety of different fish in this tank, but for the past 3 years it has been a rather peaceful community tank. I had around 10 assorted cory cats (bronze and skunk), most of which had been in the tank for over two years. I also have a blue dwarf gourami, a hillstream loach, and two glass cats. My nitrites have always been at 0 ppm, nitrates are usually anywhere from 0 to 20 ppm, ammonia is always at 0, and a slightly acidic ph. The temperature is usually always between 74F-78F. Every two weeks I do a 25% water change. The tank has been like this for almost three years. Recently, I wanted something to make it a little more interesting, so I went to PetSmart(as my old fish store has recently gone out of business) and purchased a school of neon tetras. When purchasing them, I noticed some dead neons in the tank, but did not think much of it as this is normal for chain pet stores. Each day, I noticed more and more neons would die. Another day my water was clouded, so I immediately tested for ammonia, but there wasn't any. Then my corys started to die which was very strange as they have been healthy with no problems for two years. I lost the hillstream loach, a glass cat, and my dwarf gourami has a bulging eye and does not look well. There is one neon left, and 5 corys. My water parameters have not changed. Does anyone know why this is happening? Could it be bacteria introduced by the water from PetSmart? I appreciate any responses.