So after doing a water change on my tank and adding dechlorinator I went upstairs to find 12 of my 13 fish dead, including the Nerite snail. The fish that died were serpae tetras, neon tetras, clown pleco, picture cat (gift) and then the snail. The only survivor was a white cloud minnow I’ve had for about a year now. My question is what should I do? The minnows fine and all the plants are healthy, should I add more fish soon? Start over? Tank is 15 gallons and has been running for a few years. (Btw this isn’t the first time this happened, lost six fish a few years back all at once).