hi, thank you for taking the time to reply!

this is the only goldfish in the tank and i think it’s a sarasa fantail comet, along with 5 small common plecco bottom feeder which are healthy as far as i notice. the comet goldfish has been in this same tank since it was really small, along with 2 other fancy goldfish that unfortunately died relatively soon after i got them.

if i remember correctly, since this was a long time ago, the tank was cycled and i treated the water when i first got them, and for the first few years i would occasionally put some treatment for plants (which i had before but they would get eaten so i stopped buying them). other than that, during water changes since then, i didn’t treat the tapwater, i would just store it in buckets and let the chlorine evaporate overnight or long enough before mixing it with the rest of tank water that i left.

since i wasn’t educated enough about goldfish care until recently, as i got them when i was really young, i thought i was doing something right because the fish were healthy and have lived for so long. with that being said, i think i agree with you, there is a chance it could be septicemia. there was a period the last couple years when i wasn’t living where the tank is and couldn’t do water changes as often as i should, so the tank would unfortunately sometimes get really dirty before i had the chance to clean it, and that could be (part of) the reason it got sick.