I've been in the hobby for a year now. Last November I bought two ranchu goldfish after doing as much research as possible and cycling my tank. My two Ranchu grew and were primarily healthy for a year. About six months in one of my ranchu started displaying heavy swim bladder issues. I did all I could with salts and less feeding that resulted in a tad bit better result, but alas he ended up passing 3 weeks ago. One week after my first fish's passing my original fish which was 2x bigger started swimming vertically. Why display swim bladder issues after swimming perfectly fine for a year? I figured maybe he was stressed his partner died. For the next week, he started swimming upright again and was seemingly healthy, so I went out and bought three more ranchu. Two that are 2 inches and one that was 3 inches because there was a black Friday deal (buy 2 get 1 free). Upon arriving home I had a quarantine tank prepared. I did what countless others suggested. Quarantine for 3 days, no eating, no lights, and water changes. I then introduced them into my main tank with my chonky boy. They all seemed to be fine until my oldest fish started displaying swim bladder issues yet again and within one week he had passed, leaving me devastated. Here I am now with three new fish and today I come home to find the 3-inch fish dead. Why? What did I do wrong? I just performed a water change yesterday. I perform water changes weekly at 30%, I do not overfeed, and I feed healthy greens grown from my own garden once every other week. Did the nitrates spike kill my 3-inch fish? Was my oldest fish too stressed from the addition of new fish? For context, my tank is 50 gallons and I was concerned it wasn't enough for my three fish so I went out and bought a 40-gallon wide tank to cycle and assimilate the two smaller fish into. I have a canister filter that overcompensates in gallons and 4 nerite snails in my tank as well as three underwater plants my fish don't touch. Now I'm just thinking of moving the two remaining fish that I have into my 40-gallon that will be installed with 3 filter sponges and keep the bottom bare.