I really need some answers. I’ve been a goldfish hobbyist for about 6 years now so I consider myself pretty well knowledgeable when it comes to goldfish care and illness but what happened with my batch of 3 fish that I’ve had in the same tank for 2 years has left me completely dumbfounded. I had 3 fancy goldfish in a 20 gallon tank (yes you’re thinking it’s too small but I follow the 1 gallon per inch rule and all 3 of my fish were only 1 inch long as I kept there water cold and didn’t feed them enough to grow big on purpose.) The filter I have is an aqua clear 50 and I cycled it two years ago for this batch of fish. I take great pride in the health of my fish, doing water changes weekly and always checking the parameters. I usually use prime water conditioner but I ran out and the next available thing was Tetra Aquasafe. It was brand new, used a few times for my betta fish, and it wasn’t expired. I followed the directions exactly on the container using 4 teaspoons since my aquarium is 20 gallons. My fish were perfectly fine and healthy, I went to shower, and when I went back to the tank all 3 of them were dead. I am extremely upset and very confused on how they died. The water parameters were perfect, I recently treated them for flukes, and they where very healthy fish. Does anyone know what could’ve possibly happened?? All opinions or suggestions welcome. Thank you