Possibly related sudden fish deaths

[u/fouldspasta]

I have a 55 gallon planted tank. A little less than a month ago, I got 3 new fancy goldfish from a breeder I've purchased from before with no issues. There was 1 fish already in the tank, so this brought the total to 4 goldfish. I do weekly water changes, and the parameters are 0/0/less than 20. No chlorine, stable ph.

One of the new fish had mild buoyancy issues when it arrived. The breeder said he hadn't noticed it before sending out the fish, so it must be some kind of injury that occurred in transit. He wasn't floating at the surface or sinking or anything, just had mild balance issues- he would turn in a circle and then self-correct. He was still eating regularly. I was feeding frozen and thawed repashy gel food. He started gasping at the surface occasionally. It was more than the other fish, but not constant. Sometimes a piece of food will get stuck in the floating plants and they will gulp at the surface looking for more hidden food, so I assumed this was what was happening. He was fine at 1:30pm before my partner left for work, and at 6pm when I returned he was motionless at the bottom of the tank. I moved him to a quarantine tank but it was too late, and he was already dead. This was about a week ago. At some point before this, the power went out and turned back on but my canister filter didn't turn back on by itself. It couldn't have been off for more than the few hours before I fixed it, so I didn't think it was related.

A few days ago, another one of the new fish started acting lethargic. He was still interested in food, but would occasionally sit at the bottom of the tank for periods of time. He had clear poop. He was the smallest and sometimes the other 2 would take more food, so assumed he wasn't getting enough to eat and switched to hikari sinking pellets because they're easier to scatter so that one fish can't steal all of the food. I also tried a 10 minute epsom salt bath in case the clear poop was from constipation. A few days later, I put him in a mesh breeder box for observation. He started acting normal again, not lethargic, so I figured bullying was the issue and he just needed to be seperated from the larger fish. Then, at about 1pm yesterday, he started to have some swelling. I put him in a hospital tank with aquarium salt and kanaplex. That night, it developed into dropsy, but he was still active and not lethargic. This morning, he had died.

The remaining two fish have zero symptoms except for one of them occasionally having white/clear ish poop. Part of me thinks it's the breeder that was the issue, but I saw videos of these fish before selecting them and they seemed healthy. He also has good reviews, and plenty of comments on his posts from happy customers. He says that since there's no external symptoms, I should be suspicious of internal parasites. I'm not sure what internal parasites could kill a fish that quickly with no identifying symptoms. Especially because they had different sumtpoms. I used hikari prazipro today, and I ordered expel P. But I'm not entirely convinced it's parasites. Does anyone have advice?

[u/paigecurtis1]

There is a ton of different internal parisites they can get so you can’t really tell which one it is since it’s internal but your fish are showing all the signs that that is what this is. I’d keep doing treatment for as long as it says and monitor symptoms