Cardinal Tetra deaths

Woke up this morning to 8 or 9 dead cardinal tetras. Discovered another few when I got home.

The tank is 75 US gallons, and houses 6 discus, 25ish (probs 15 now) cardinal tetras, 20ish rummy nose tetras and 3 blue rams. Yes it’s over stocked, but it is well overfiltered and heavily planted. Everything has been pretty good the past 2 years.

All that has changed has been this week. I normally have the lights on way too long. I work 3pm til 11pm, and tend to flick them on before leaving work so I can enjoy the aquarium when I get home without shocking the fish turning the lights on at night. After some algae issues, I have cut back on feeding, and elected to leave the lights off. This means the tank has only had natural sunlight throughout the day. I am wandering if the discus are going after the cardinals at night (especially as I have cut back on feeding)

The only confusing thing is the Rummynose have been left alone, despite being much much smaller than the cardinals. All cardinals are thick and fully grown, the rummy nose are relatively new to the tank.

I have also left the discus for 8 days with no food/light etc when on holiday, and this never happened.

Any ideas?

Just ideas -nothing concrete. I have experienced things like this, with one species dying off suddenly. I have even seen it in 2 different tanks, at the same time, which has always perplexed me.

If the Discus decided to go for it - cardinals aren't as fast or as well open water camouflaged as rummy nose are.

Short term deprivation is a breeder trick. You cut back on food, light or water changes and the fish will sometimes decide the end is near and get breeding. Do you think you might have triggered the rams? Cardinals wandering close to their breeding territory are fair game to them. That said, one or two cardinals would make sense. You've lost a lot.

It sounds more like disease though. What disease, who knows. Viral? Bacterial? All we have are guesses.

How new are the rummies? I have lost species when they were wild caught (as cardinals usually are) and I have added a plague bearing farmed species to the tank. The farmed fish survive, but the things they carry can be fatal in a first contact way.

Are the cardinal tetras disappearing or are you finding entire or parts of bodies?

Pictures of the dead fish?
Pictures of the remaining cardinals?

How long have you had the cardinals for?

What did you do to the aquarium yesterday?
Did you have visitors yesterday?
Have you added anything to the tank in the last 2 weeks?

Are you adding carbon dioxide (CO2) to the aquarium for the plants or to lower the pH?

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If you put the lights on a timer you can see the fish when you get home and don't have to worry about turning lights on and off. I ran two lights on timers in my fishroom. One was a low wattage light that came on first and added light to the room. About an hour later the tank lights came on. At night the reverse happened and tank lights went off, then an hour later the room light went off.

GaryE said:

How new are the rummies? I have lost species when they were wild caught (as cardinals usually are) and I have added a plague bearing farmed species to the tank. The farmed fish survive, but the things they carry can be fatal in a first contact way.

Click to expand...

That was my first thought too

GaryE said:

Just ideas -nothing concrete. I have experienced things like this, with one species dying off suddenly. I have even seen it in 2 different tanks, at the same time, which has always perplexed me.

If the Discus decided to go for it - cardinals aren't as fast or as well open water camouflaged as rummy nose are.

Short term deprivation is a breeder trick. You cut back on food, light or water changes and the fish will sometimes decide the end is near and get breeding. Do you think you might have triggered the rams? Cardinals wandering close to their breeding territory are fair game to them. That said, one or two cardinals would make sense. You've lost a lot.

It sounds more like disease though. What disease, who knows. Viral? Bacterial? All we have are guesses.

How new are the rummies? I have lost species when they were wild caught (as cardinals usually are) and I have added a plague bearing farmed species to the tank. The farmed fish survive, but the things they carry can be fatal in a first contact way.

Click to expand...

I don’t think it’s the rams, lost another 2 today. Can’t imagine they could take out over 10 fish in a couple of days. The cardinal are chunky as well, probably bigger than the rams.

The Rummynose tetras were added maybe 3/4 weeks ago. No fish showing any signs of illness or distress. No aggression present either

Colin_T said:

Are the cardinal tetras disappearing or are you finding entire or parts of bodies?

Pictures of the dead fish?
Pictures of the remaining cardinals?

How long have you had the cardinals for?

What did you do to the aquarium yesterday?
Did you have visitors yesterday?
Have you added anything to the tank in the last 2 weeks?

Are you adding carbon dioxide (CO2) to the aquarium for the plants or to lower the pH?

-------------------

If you put the lights on a timer you can see the fish when you get home and don't have to worry about turning lights on and off. I ran two lights on timers in my fishroom. One was a low wattage light that came on first and added light to the room. About an hour later the tank lights came on. At night the reverse happened and tank lights went off, then an hour later the room light went off.

Click to expand...

They are not disappearing; I am finding the bodies.

Nothing has changed in terms of visitors, addition to the aquarium or change in routine.

The live cardinal tetras look fine and don't appear to have a disease.

The body has been chewed on by something but they only ate the guts so that happened after the fish died.

If it was a water quality issue it would affect more than just the cardinals.

I doubt it's a disease because they look healthy and you haven't added anything new.

This leaves aggression/ predation. The bodies aren't being eaten completely so presumably it's aggression based.

Are the cardinals avoiding an area?
If yes, who lives there?

If you cut back on feeding, maybe increase feeding to make sure nobody is eating them at night. Fish that are well fed are also less aggressive when protecting territories because they have sufficient food in their bellies.

Sure thing - my thoughts were the discus. The cardinals normally shoal throughout the front of the tank but they have been relatively stationary on the left hand side of the tank behind the plants. The discus pair that breed frequently lay their eggs on the right hand side of the tank.

My only confusion is why now? They have always lived harmoniously - why would they suddenly kill so many cardinals all of a sudden?

I once had some Satanoperca that lived peacefully with some lampeyes, even through breeding. Then one day while I was at work, they killed (but ate) the entire shoal. I assume it had just never occured to them, and once they somehow tried one, they liked it. Whatever triggered the first killing sent them on a hunt. They aren't an especially predatory fish.

Aquarium levels of crowding produce weird behaviour. We want things to be as natural as possible, but even big tanks aren't really big.

I still think something could be going through the cardinals. But if it is, it's nothing you can treat.

Confirmed to be the discus. Witnessed them tonight hunting and killing the tetras. They managed to take out 2 of the rummy nose as well. I guess at this point I have no choice but to rehome the tetras.

Such a shame as I’ve worked towards being able to keep a planted display tank of discus and cardinal tetras for so long, my favourite fish and combination

Have you lowered feeding the Discus ? If they are eating the guts of the victims it's to steal nutrients.

It's not really recommended to house small tetras with Discus. But If you keep them well fed and full. They should not start to have bad ideas.

Once they started, it's nearly impossible to bring back, sadly.

Well, your tank is beautiful. Hate it that the combo won't work anymore, but even without the tetras it is such a nice tank.