That's a goldfish right? It looks like fin rot from your picture. Is there anything else in the tank with him?
Cory and pleco
What are your water parameters?
I never tested the water and how do I make the fish less stressed
I hate to break this to you but cories and plecos and goldfish can't really go together. Goldfish need cooler water where as the other two are tropical. Also, cories are schooling fish and need to be in a group of about 6, which would probably explain the stress. How big is your tank?
Stress is caused by aggressive tank mates, overstocking, elevated ammonia/nitrite and many other factors. The first thing to do is to find the cause of stress. Can you post your tank size, try and buy a testing kit to check parameters that will help as well. How often do you do water changes?
Hello fish... Goldfish don't belong in a tropical fish aquarium. The water is too warm. These are cool water fish and don't even need a tank heater. Tropical aquarium fish do. Likely the reason for the fin ailment. These fish get really big and are heavy waste producers. Unless you have a very large tank, the waste material from one Goldfish will cause sudden changes in the tank water and stress the fish. Probably another reason for the fin ailment. You have a water problem. The tank needs a large water change asap. Goldfish need their own, very large tank. Old
Goldfish get big, you need a big tank for them. Plecos get big, you need a big tank for them. Cories are small fish, but they love to explore and they need a lot of ground to cover, you need a big tank for them. So I'm hoping you have a big tank for them. Secondly, Plecos are tropical fish, they require 72-80 deg Fahrenheit water. Cories are tropical fish, they require 72-80 deg Fahrenheit water. Goldfish prefer cooler water and this water is much too warm for them. You starting to see my point here? You haven't even tested the water, even more reason for fin rot to show up. Plecos are big waste producers, you must clean the tank often. Cories are minimal waste producers because they're tiny, but if they have plecos as tankmates, it'll need to be cleaned often, and even if they don't have plecos in there with them, you still need to keep it clean. Goldfish are massive waste producers, the tank needs to be thoroughly cleaned frequently. You need to switch the goldfish to a large tank as soon as possible and do an at least 50% water change in the pleco/cory community tank. You may gradually build up the percentage of water change you do in the goldfish and pleco/cory tank, but you need to build up the water change in the goldfish tank to at least a 75% weekly. 20-25% monthly is what most LFS's will tell you, but that's just inefficient for something that produces as much waste as goldfish do. As soon as you have completed these necessary steps, you must treat the fin rot as soon as possible. Welcome to the forum. Wishing the best for you and your fishies!
Thanks so much
What is your tank size and do you understand the nitrogen cycle? You NEED a test kit to keep fish. Without water parameters we can't do much to tell you what's wrong. What we can deduce is that the water is likely warmer than the goldfish needs it to be and if you've never tested the water, it's likely polluted. Especially if your tank is smaller than it should be.
No problem! Sorry if I seemed a little irritated in my post, I just get so ridiculously upset over fish if they don't have the 100% correct care. Wishing the best for you and your fishies!