Help! What did my catfish die of and is it contagious?

[pkenziep]

First off - I apologize as this will probably be a novel.
Can anyone help me ID what killed my albino corydora? I've had two for over a year (which I know they need more friends, but I didn't have tank space until very recently and haven't even had a chance to increase their school size), and they've been extremely healthy and happy fish.
I kept them in a 10gal with 7 neon tetras - then moved across the country and upgraded to a 29gal tank (put them in ~ 8/27/2020 once the tank had cycled). The following week I added an assortment of fish I took from someone who was going to just throw them out (I quarantined them a week). I have a total of 25 fish, which are small schools of tetras, minnows, and lambchop rasboras - I know it's at max capacity but I can't get a bigger tank. I may put a school of tetras in my 10gal to make some space. Everyone has been doing great (even in the old tank!) until now.
Last night I noticed the one catfish didn't eat at feeding time and he seemed a bit lethargic. I wasn't too worried but I still checked on him first thing this morning. He was extremely lethargic and had a reddish-purple bruise-like appearance on his head (which I hadn't seen yesterday). His scales seemed puffed out in a way that resembled dropsy. I'm looking back at pictures I took of my fish on 9/24 and he looked completely healthy. I'll include pictures of the bruise-like thing but I apologize they're not good quality. Albino catfish don't photograph well!
I checked the ammonia which was shockingly high. It was 0 less than a week ago but today read at 2-3ppm (). It's a planted aquarium but I always remove dead leaves as soon as I see them and I don't overfeed my fish. I have a fair amount of shrimp in case there is food leftover. This new tank has fluval stratum substrate instead of the gravel I used in my old tank, if that makes any difference.
pH 7.0
Nitrite 0
Nitrate 20 (it's usually 0 so that was a surprise too)
I added Imagitarium ammonia reducer, did a 50% water change, and changed the biological material in my filter. I treated my tap water with API StressCoat. I also added Melafix. I threw in an almond leaf for good measure.
A while after doing this, I went grocery shopping, and, by the time I got back, he was dead. When I took him out of the tank, I was able to see his underside also appeared bruised, and he had a small bright red bloody-looking spot on his belly. It was about the size of a small picture-hanging nail head, if that makes sense. I apologize I don't have a photo of it, dead fish creep me out. If you need a picture I can get one (he's in the freezer so he can have a proper burial because I'm that type of person).
Because of the bloody-looking spot, I've been panicking and googling to an exhausting point. I can't find many examples to look at but it kind of looks like it may be red spot disease? Or possibly septicemia? I'm clueless.
Bottom line - ammonia shot up, corycat quickly became lethargic, developed bruise-appearance on head and belly plus a blood-red spot on belly, scales turned spiky, and ultimately death.
Any ideas what happened? Everyone else in the tank seems completely healthy and active, including the other cory, but, knowing how quickly their health can change, I'm extremely worried. And if it is something contagious.. what should I do? Besides a brief spell with ich, I haven't had illnesses in my community tank, and I haven't had any deaths (I've had the community tank about three years). I'm very experienced with treating ill bettas, but only in solitary tanks.
Thank you so much in advance for your help. And please be kind if you can, I'm having a rough week.

 

[AdoraBelle Dearheart]

I'm sorry for your loss

May I just clarify something please? How many fish did you add when you took in those rescues?
Because it sounds as though you had nine (two cories, seven tetra) then added enough fish that it took you to 27 total? That would explain the ammonia reading. It was just too big a jump in the bioload at one time for your filtration and plants to keep up with, leading to an ammonia spike.

It sounds to me as though he died from ammonia poisoning, I'm sorry If your ammonia test is accurate (which test kit do you use? The dip strips aren't always accurate) but if it was really that high, it's fortunate that most of the fish have survived. I hope that doesn't sound harsh, I'm not judging you at all! It happens, and it's very sad.

For the future though, anytime you add new fish, best to quarantine them for a few weeks first to make sure they're disease free, then add them in smaller batches at a time where possible. Of course, you were taking in fish rather than buying them, so having such a big group at once wasn't by choice. When you have to do something like that, you're likely doubling or tripling the bioload of the tank, and it takes the bacterial colonies some time to catch up and grow enough to consume all the extra ammonia being produced. So ammonia spikes happen, and fatalities happen.

So if you have to add a big group of fish at once, you'll need to test the water daily and likely do large water changes daily. The water changes dilute the ammonia, so if you only change 50% of the water and the ammonia is at 3ppm, you're still leaving 1.5ppm ammonia in there - way too high still for fish! So better to do a 75% water change to dilute it further, then repeat, until ammonia and nitrites are back at zero. Products aren't as reliable as clean fresh water.

Again, I'm very sorry for your loss.

 

[AdoraBelle Dearheart]

Serves me right for answering before finishing my coffee - just checked the dates you mentioned, so you added extra fish around September 4th, is that right? And ammonia stayed at zero despite being a new set up and a lot of new fish added, and only spiked yesterday?

In that case, it wasn't the increased bioload, but it's odd that a new tank (unless you used the same filter, substrate and plants and they didn't dry out) didn't have a mini cycle at all when first set up, and adding a lot of new fish didn't cause a mini cycle either, do you test the water often? How often do you do water changes?

Check the tank for the source of ammonia. A dead fish, snail or shrimp is the most likely culprit for ammonia rising that high.

Why did you throw away the bio media? I'm afraid that may have done more harm than good, since the beneficial bacteria we need to process ammonia and nitrites live on the biomedia (among other places), so replacing it means you've thrown out a good chunk of the bacteria needed to handle that ammonia. You're likely going to need to keep on top of testing and daily changes until the bacterial colonies can grow back to previous sizes.

Still sounds as though he died of ammonia poisoning to me, since ammonia was so high, but the cause of the spike wasn't as I thought, given the dates. @Colin_T , any thoughts?

 

[Barry Tetra]

Water qualities problem; either Ammonia or Nitrate poisoning...
do 75-90% daily water changes and substrate clean, make sure there are Oxygen and No Chlorine and Chloramine before added in


also you’re not the only one who put dead fish in a freezer.....I have like a thousand of Tiger barbs in the freezer right now...

 

[pkenziep]

Serves me right for answering before finishing my coffee - just checked the dates you mentioned, so you added extra fish around September 4th, is that right? And ammonia stayed at zero despite being a new set up and a lot of new fish added, and only spiked yesterday?

In that case, it wasn't the increased bioload, but it's odd that a new tank (unless you used the same filter, substrate and plants and they didn't dry out) didn't have a mini cycle at all when first set up, and adding a lot of new fish didn't cause a mini cycle either, do you test the water often? How often do you do water changes?

Check the tank for the source of ammonia. A dead fish, snail or shrimp is the most likely culprit for ammonia rising that high.

Why did you throw away the bio media? I'm afraid that may have done more harm than good, since the beneficial bacteria we need to process ammonia and nitrites live on the biomedia (among other places), so replacing it means you've thrown out a good chunk of the bacteria needed to handle that ammonia. You're likely going to need to keep on top of testing and daily changes until the bacterial colonies can grow back to previous sizes.

Still sounds as though he died of ammonia poisoning to me, since ammonia was so high, but the cause of the spike wasn't as I thought, given the dates. @Colin_T , any thoughts?

Yes I did quarantine the others and I actually did add them in small batches over a week. Maybe that was too quickly?
I do a five gallon change about once a week and I test the water twice a week.
I changed about half the bio media because I knew I needed to keep some but I guess in my panic I made a bad decision. I got this filter when I moved and it’s my first time using a filter media basket rather than the basic Aqueon cartridge type. I’ll do more research to learn about it. Thank you for the reminder! When I started the tank I added old water from the tank back at my old home.
I did check for dead fish etc but didn’t see any! I’ll move some more decorations and try again today. It’s so weird! The speed of the ammonia spike just baffles me.
Thank you for your help!

 

[pkenziep]

Water qualities problem; either Ammonia or Nitrate poisoning...
do 75-90% daily water changes and substrate clean, make sure there are Oxygen and No Chlorine and Chloramine before added in


also you’re not the only one who put dead fish in a freezer.....I have like a thousand of Tiger barbs in the freezer right now...

Thank you! That’s what I plan on doing. I appreciate your input.