I don't like how quickly that fin went from a white spot, to engulfed. But it doesn't look like any fungal thing I've seen either. I'm suspecting that the fin/barb on that side was damaged when he was caught at the store or somehow did it in the tank, and there's infection or a lot of excess mucus because of the injury. Adding the skinniness to the equation as well though...
I'm far from a disease or injury expert though, that's just my suspicion. I'd still follow Colin's advice about getting some aquarium or rock salt and trying that. I've seen salt treatment knock out an infection in 2-3 days. Certainly worth a try if you can get hold of some of the right salt soon.
I'm definitely a little panicky! Could you tell? I'll be honest I'm not experienced in big /community tanks whatsoever. I've kept 5gal betta tanks before. No live plants, single fish per tank.
I definitely understand and remember the worry when starting out in the hobby and trying to do the best you can, how steep the learning curve is, and it's tough when something goes wrong since we can't really practically rush a cory to the vet the way you would if it were a dog or cat! We have to puzzle things out for ourselves really. But I also think following the advice Colin gave about tank cleaning, water changes and using salt water treatment will give him the best chance. Colin does know his stuff, and I've used salt treatment with success before too.
But you're doing fine! You managed betta tanks, you can manage a community tank. It needs a bit more planning out with stocking and plants etc, but also easier in many ways since larger tanks means more water dilution, and more time to catch and fix issues than in a small tank. Keep on top of maintaining the tank in terms of cleaning the substrate/water changes/filter cleanings when needed etc, and you should be fine!
I'm very afraid of just being another ignorant fish killer.
Sadly, sometimes fish do die in this hobby, it's inevitable really, and sometimes it happens because of a mistake we've made. I know I've made more than one mistake that has killed fish, and the guilt is horrible. But you resolve to learn from it and improve your hobby in the process. I know I was doing my best to give the fish everything they needed, but have made mistakes from not knowing something, but it was accidental, not callousness or wilful ignorance, so you have to forgive yourself and move through and past these hurdles that we all face when we dive into this strange and wonderful hobby!
You're clearly not a callous fish killer who doesn't care, or treats them as a kid's toy or decor, not caring when they have to replace the goldfish in their bowl every month. You clearly care a lot, willing to seek help and do what you need to try to help the fish -no one can ask any more of you! That shows your heart is in the right place, and you've also come to the right place to get advice for treatment and any questions you have about your community tank.
This is also unlucky by the looks of things, not something you've done wrong. It's bad luck, but even if you do lose this particular fish (and I'm not saying you will, have seen fish bounce back from much worse!) it isn't your fault, and you can still keep a beautiful and peaceful tank.
This particular fish has always behaved in a "different " way. When I bought them I didn't have any experience in inspecting them other than to avoid tanks with ick or gasping/ dead fish. But I did notice immediately this one always had a darker color, & was "shy". With feeding, only one out of the group recognized the sinking food straight away. The others eventually got to it, except this one. I'm attaching a picture of one of his group. Who has behaved as
" healthy ". He's relaxing in his favorite spot, but not behavior is schooling, rummaging etc.
It's very easy to accidentally get a fish that isn't quite right when you're buying a batch and the store person is bagging 5-6 or more fish for you from a batch of 25, and they looked good in the tank. I've wound up with a few duds that way too, poor guys. Rescues too, because I'm daft!
I haven't kept this particular species of cory, so I might be wrong about him being a runt, that might just be the skinniness and the fact I'm used to different shapes in my own cories that's throwing me off. But I have bred bronze and pygmy cories, plus guppies/mollies/platies, and when cory fry grow, it's surprising how they can grow at really different rates. I've raised 4/5 (I honestly can't remember!) batches of bronze cories, the last batch was large, with 30 odd babies, and I was surprised when one of the lot grew to twice the size and weight of all the others! Fish raised in exactly the same conditions/water/food availability, but some will be faster, stronger, more active in searching for food, and it's also common to have some slower, unhealthy fry that pass away, and sometimes runts that just aren't as strong and healthy as the rest. You lose some fry that clearly didn't develop properly somehow, and have some that are slower to grow.
One of the bronze cories I bought had a kink in his spine. A minor kink that is almost impossible to see from the side, I didn't even see the kink until after they'd had fry and I was moving them to another tank, and I saw the spine kink when he was in the bucket and I could see him from the top. It's a deformity, minor enough that the fish doesn't show any ill effects, but the group did produce two much more deformed fry that I've kept so they can live out their lives in my tanks, and I no longer let the bronzes breed/don't save any eggs, because I don't want to have that deformity pass on into the hobby. So I might be wrong, it might just because he's thin and unwell, but as Colin said, he might have something wrong internally/be weaker than the others, especially if he is a runt, and you were just unlucky that you're the one that got him.
In the photo, the one at the back was born with almost no tail, just a tiny strip, is much smaller than a normal adult, but still potters about and survived so I've kept him. The other has a much more kinked spine and also remained much smaller, I didn't expect them to survive since their deformities were obvious from when they were small, but I'm happy to keep them with my other bronzes, not breed from my bronzes anymore, and I expect these two will likely have a shorter lifespan since they didn't develop properly. It happens sometimes, sadly. But they potter about with the other fish seeming normal enough otherwise.
I've decided not to move the laboring platy. I will just try to catch some fry & relocate them. I'm pretty sure they'll mostly get eaten. They have so far. Thank you for taking some time to respond & trying to calm me down. I need that!
Another tip to try to get to fry before the bigger fish do - when a female is looking ready to pop, in the mornings check the tank with a torch. Don't put the room light or tank light on, just check the surface/plants/substrate for fry using the torch. If they've been born, then you can scoop them out with a net before the other fish wake up. Check for fry again before going to bed. If you add a mass of dense floating plant, like elodea or hornwort, left to float in a dense tangle at the surface, the fry will instinctively head to that to hide, and can hide remarkably well from the adults.
The plants above are Amazon Frogbit, a really easy floating plant. Just throw some in and it reproduces like mad, and grows those long roots which are great for fry to hide among, and dodge behind to escape a hungry adult. Pretty much any floating plant (except duckweed, avoid duckweed, it's evil) or stem plant like anacharis or hornwort will work, and the latter two are cheap and easily available. Just throw it into the tank and let it float, and makes a perfect nursery for fry to hide in, and you can choose to catch the fry and put them in a breeder box or nursery tank if you choose.
Thank you for taking some time to respond & trying to calm me down. I need that!
Happy to do it! Sorry about this long essay reply, anyone here will testify that I'm a waffler. Let me know if anything is unclear or you have any other questions. Someone here is bound to know the answer, and we're generally a friendly bunch!
