Sudden Cardinal Tetra death

Hi everyone, not very happy to be posting here today, but alas it’s part of the hobby to learn.

Literally overnight, and honestly over the span of a few hours it seems like, one of my cardinal tetras sadly died. I walked past the tank this morning (although, I’ll admit I didn’t pay TOO close attention so I may have missed him) and there was nothing suspicious. Few hours later, I see a pale cardinal tetra floating on top, unable to move, and quickly opening and closing its gills. I quickly moved it into a breeder box for closer observation, and I watched as it quickly passed. It was all so sudden. I’ve had the cardinals for about 7 months.
I fed everyone the night before, nothing suspicious at all. Everyone was voracious and happily ate. I did notice a slight behavior change of one of the cardinals (I’m assuming it’s the one that died), kinda stayed away from the group a bit but still ate to it’s hearts content so I thought mothing

One thing I noticed when the fish passed is that only the back part was pale and it had a strange black line on it’s underside. I also saw some nearly invisible, strange thin liquid of some kind slowly leaking out of its anus (sorry to be gross). Pictures attached. No other fish has symptoms or behavior changes.
I’m at a loss as to what this could be, any advice is seriously appreciated. I’m so worried my other fish will catch whatever this guy had and pass as quickly and suddenly as he did. My tank is planted and has shrimp colony and a snail so my treatment options are also limited.

Only change that I noticed in the tank within the last few days is that I woke up today to find my drop checker on the gravel, with some of the liquid gone. I’m wondering if maybe the cardinal ingested the liquid?

No new fish has been introduced in months.

Obligatory info:
Tank
What is the water volume of the tank? 29 gal
How long has the tank been running? Over a year
Does it have a filter? Yes, 2
Does it have a heater? Yes
What is the water temperature? 78-80
What is the entire stocking of this tank? (Please list all fish and inverts.)
5 Cardinal tetras (I have been having issues with the cardinal stock in my local area, I’m trying to increase the numbers of the school but many have died suddenly or from columnaris in quarantine. I’m wondering if what my guy had was related to this)
4 red honey gouramis
3 endlers
Shrimp colony
1 nerite snail

Maintenance
How often do you change the water? Once a week to once every other week
How much of the water do you change? 30%
What do you use to treat your water? prime
Do you vacuum the substrate or just the water? Just water

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? API master kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0
Nitrite: 0
Nitrate: 20
pH: 6.8

Feeding
How often do you feed your fish? Once a day, skipping a day or two a week
How much do you feed your fish? Between 1-3 small pinches
What brand of food do you feed your fish? Hikari fancy guppy, big bites, occasional bloodworms
Do you feed frozen? once a week bloodworms
Do you feed freeze-dried foods? No

Illness & Symptoms
How long have you had this fish? About 7 months
How long ago did you first notice these symptoms? Sudden
In a few words, can you explain the symptoms? Read Above
Have you started any treatment for the illness? No
Was your fish physically ill or injured upon purchase? No
How has its behavior and appearance changed, if at all? No

 

From my experience, suddenly death can happen in genetically weaker fish very often, especially with fish like neon tetras. I don't think it was anything wrong with the tank either as it looks beautifully planted. Sometimes in the aquarium hobby, however, there are times where you just can't know, and I think this is one of these times. Very sorry for your loss, cardinal tetras are a great fish, hope the rest stay healthy.

 

From my experience, suddenly death can happen in genetically weaker fish very often, especially with fish like neon tetras. I don't think it was anything wrong with the tank either as it looks beautifully planted. Sometimes in the aquarium hobby, however, there are times where you just can't know, and I think this is one of these times. Very sorry for your loss, cardinal tetras are a great fish, hope the rest stay healthy.

Yeah, when it is like this, most of the time it is the breeder and you getting a bad batch.

 

Thank you both for your kind messages. I’m keeping an eye on my tank still regardless, not feeding for today. It’s just so discouraging that you can do everything in your power to give them the best life and stuff like this can still happen.

Do either of you see anything suspicious on the attached pictures?

 

Yeah, when it is like this, most of the time it is the breeder and you getting a bad batch.

Do you think it could still be weak genetics even if it's months down the line?

Thank you for sharing the video, it makes me feel better but I still feel uneasy about the situation because for him it seemed to happen right after adding the fish, not much later.

 

Do you think it could still be weak genetics even if it's months down the line?

Thank you for sharing the video, it makes me feel better but I still feel uneasy about the situation because for him it seemed to happen right after adding the fish, not much later.

Well, it's all about perspective. The fish may have died right after you got it, but that doesn't mean it was your tank that caused the issue. These cardinal tetras may have been dying of for months at the breeder or lfs, you just didn't know it.

 

That's true. I have struggled over several batches in the course of a year to keep any of them alive right after buying (I've gone through around 25 fish, only to have my 5, now 4, remaining). I thought the 5 that I had were the hardy ones. Guess the weak genes still won.
I guess I'll have to source my cardinals online from now on. Don't even know where to find a reputable cardinal breeder online or in my area. I'll need some time though in case there is a mystery illness. I'll keep this thread updated on if there are any other fish showing symptoms.