Torn fins of a Telescope Goldfish

Hello!
First of all let me clarify a little bit. English is not my native language, it's my first time posting here and it took me a while to find the proper (if that is) section to post such a help cry.

So I'm a proud owner of 4 calm fish. Two of the catfish family, two types of goldfish.
My smallest one, Mituna, a black telescope goldfish had his scales damaged. To be specific it looks like he has a broken analfin, left pelvic fin and left pectoral fin.

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No one else in the tank has ever demonstrated any kind of aggression especially not to him so I am positive it's not fin nipping. He usually calmly swims near the top corners but lately he has been laying near the bottom and very close to the filter. I've been worried he might've got his fins damaged physically because I haven't detected any signs of fin rot. My other fish also behave normally and haven't ever been sick. I'm changing the water regularly and feeding them a small dose twice a day. There is a lightning cycle I'm keeping so they won't get too tired by the light. I change the water oft-times enough and I don't keep any dangerous equipment or any plants. The only thing that got me worried is the actual filter. I've got an aquael filter. His feeding habits remain the same and he's as social as ever. I've had a fin problem before with one angelfish but it wasn't serious and he "grew" it back and it looked as fine as ever. But as of the past month its only been getting worse and I am not very patient when it comes to the health of my pets.
Any help? Should I worry?

Thank you for reading!

EDIT: Phasing.

 

Welcome to Fish Lore!!! I'd say you did a pretty good job with your English!

What are your water parameters?

What's your tank size?

What test kit are you using, if any?

What exactly are the other fish being kept in the tank?

 

It should be about 10 gallons if I did my math correctly (about 40 liters)
I keep small rocks instead of sand on the bottom and there are a few bigger ones that aren't sharp.
I've got a spotted hoplo catfish named Antonio, Goldfish named Hector and an ancistrus, Gamzee.
They've been living together for almost two years now, too. Update on the situation. Overnight the tail fin looks like its almost gone and I can see some red on the back. Not blood but probably the tissue. I don't have a test kit and I don't know much of the water parameters. The water temp is mostly around 22c, the pH of the water I'm using is around 8 but I don't know what it is in the tank. :/

EDIT: I've been thinking of a proper test kit to try out but they're kind of expensive so I might have to wait that out for my salary.
EDIT2: I've been thinking about something. Since Black moors have poor eye-sight they need more time in order to eat their food. (I'm feeding universal flakes to mine) Mituna has been growing slowly, whereas Hector, my big goldfish has been growing a lot more and at a rapid rate. Do you think keeping them in the same aquarium is a good idea at all? Is Hector eating most of the food of Mituna? I don't have an option to keep them seperated however.. so it goes down to that. I won't be pairing black moor fish with other types of goldfish anymore. I don't think it's a good idea.
EDIT3: Update. I'm home from work and his fins aren't worse but he's swimming upside down in the upper corner of the tank.

Could it be a bladder problem? He hasn't done that before.

 

Okay, first off, that tank is way, way too small to house all of those fish, I'm afraid. Goldies are high waste producers, as is that ancistrus (also known as a bristlenose pleco, or a BN pleco). The tank isn't going to work for the hoplo catfish long term, either.

It is possible, that due to poor water conditions, your goldie has developed fin rot. But it is nearly impossible, imo, to keep your water conditions in good shape with your stocking, sorry to say. But I can't say this for sure until I know your water parameters. The API Freshwater Master Test Kit is the most often recommended one on here, it is around $22 in US dollars.

As for the swim bladder, peas are often a good bet, but I'm not sure about feeding them to goldies, or if it even makes a difference what kind of fish they are. @Ibonini1 might be more knowledgeable in this. But I did find this link online:

Like I said, tank parameters would be greatly helpful in this situation.

 

Your English is better than most native English speakers'.

 

I was afraid one might say that for the size.They've always seemed to have enough space although this is quite subjective and easy to say from outside the tank. I've been wondering, how do fish live through their full life span in the tanks in the shops when they don't get sold when they're so cramped? ???

 

TMK, they don't typically live their full lifespan in a store tank. I think they only spend a few weeks at most in those tanks, and no, it's not a comfortable existence. They're like dogs in cages at the pound. Yes, they can survive in those tanks, but they can't really live in those tanks, if that makes sense. It isn't a spacious place for them to be at all. Plus, I'd say that many (if not most) have seen a dead/dying/seriously ill fish in a store tank.

 

Just testing the reply since it doesn't seem to be working.

Alright, it's working now. Rewriting my response.

So, yes I'm not implying all are living through their full life span but I've seen some rather happy fish and old fish living their last days in a cramped yet clean tank in a store. What you said made total sense in my head though, don't worry So, for the time being I got advised to separate Mituna. All I have is a few liter fish bowl that I haven't used. Don't know if I should risk and move him there to keep him in his tank?

Thanks for all the help, thanks to everyone.

Is there something I can do at the moment to ease out their lives?
How big should a tank be for two catfish and a goldie to live well? I've heard its 20 gallons per goldfish but this seems way too much.. I'm not a fan of smaller fish but I don't think I'll be capable of cleaning it on my own if I mange to buy a bigger tank. I really don't want them to live the rest of their years feeling caged this badly. :/ I'm guilty.