55 Gallon Tank 15/23 fish have died in a week in my established tank, help

All of these fish have died in the last week without symptoms:

4 corydoras
5 glowlight tetras (new)
4 neon tetras
2 phantom red tetras (including my oldest fish, who survived a horrible camallanus infestation and the resulting levamisole treatment that wiped half of my tank about 5 years ago- she was my sole survivor from my horrendous early fishkeeping days)

What I have left:

1 corydora
3 glowlight tetras (new)
1 neon tetra
1 dwarf Pearl Gourami
1 hatchetfish
1 sailfin pleco

All the deets:

55 gallon tank. Lots of live plants all thriving. I moved 3 years ago which messed up my nitrogen cycle but after a few months it re-stabilized and I haven’t had a single problem in about 2.5 years. Probably lost two fish in that time, I reckon from old age. The tank has been super low maintenance (all I have to do is water changes and it thrives). I have a large Cannister filter. Keep the tank around 76. I treat new water with SeaChem Prime. Automatic lights with 24 hour daylight cycle (sunset-daylight- evening light- light off overnight)..

On February 22nd, I decided on a whim to go grab some new tetras and plants as I hadn’t added anything new in months. I got 8 glowlight tetras and 3 new plants (PetSmart). I did NOT quarantine these fish (which I’m severely regretting now) and just added them to the tank.

All seemed fine and all fish looked healthy and happy for about a week. Then I noticed one of my corydoras had died. I thought it was a one off. Then I noticed another had died. At this point I was suspicious and gave all my fish a visual exam. Everyone looked happy and healthy. All active, no emancipation or bloat, no faded colors, no skin lesions, nothing. I kind of shook it off and assumed everything was fine. Didn’t pay close attention to the fish this week tbh because I’ve been busy due to our new puppy (just dropping in their food in late at night when it’s too dark to see them). Tonight I finally sat down and did an inventory since I noticed it was looking sparse and was horrified to realize over half of my tank inhabitants have died since last week (when I did my last visual inventory after the two corydora deaths).

I am absolutely devastated as many of the fish who died have been with me for over 3 years. All of the remaining fish look totally fine, but so did the others before they died.

I tested all my levels (see results below) and nothing in there can explain the sudden mass deaths. From Googling it seems like it could be environmental poisoning, but I have no clue how because we don’t use sprays, scented candles, air fresheners, and nothing has changed in our home or their tank other than the new plants and the new fish. I am a fanatic about making sure no soap or detergents or other chemicals go anywhere near my tank. But how could aquarium plants or new fish cause mass sudden deaths either?? I know new fish can bring in lots of diseases but I would think I’d see symptoms rather than fish just dying out of the blue??

Unfortunately I wasn’t able to see their bodies as the resident pesty snails devoured their poor little bodies before I even noticed they were gone. I did look over the two dead cory cats when I pulled them out but didn’t see anything out of the ordinary.


Tested with API Liquid Test Kit (pics attached):

PH: 7
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: Elevated but I think in the 40-80ppm range (see pic), but I honestly think that’s because so many fish have died in the last week, and also the fact I’m sure lots of food was left uneaten this week due to a lower # of living tank inhabitants. I’m also about two weeks overdue on a water change. I do 15-20% water changes (with minor gravel vacuuming) pretty consistently every 1-2 weeks but again, with new puppy was busy and put it off. Even if nitrates were the problem, they wouldn’t be causing sudden symptomless mass deaths like this would they?

My current plan is 25% water changes every few days until my nitrates are back down (don’t want to lower too quickly and cause nitrate shock), but I have NO clue what else to do Do I remove the plants I added (see pics- all were advertised as aquarium plants)? Do I humanely euthanize the 3 remaining glowlights despite them appearing healthy in case they were the source?? I have no clue what the problem is (new plants? New fish? Some kind of environment poisoning? Should I treat for bacteria? Parasites?) so have no clue how to treat it, and I am TERRIFIED I’m going to lose my whole tank. My heart fish, my sailfin pleco, is thankfully still alive and appearing to be doing well, but I will be absolutely devastated if I lose her and all my other fishy friends on top of the 15 I have already lost. I feel horrible that I haven’t been paying close attention to them this week because of the new puppy and wish I had noticed and intervened sooner.

I would be so grateful for any advice.

 

All of these fish have died in the last week without symptoms:

4 corydoras
5 glowlight tetras (new)
4 neon tetras
2 phantom red tetras (including my oldest fish, who survived a horrible camallanus infestation and the resulting levamisole treatment that wiped half of my tank about 5 years ago- she was my sole survivor from my horrendous early fishkeeping days)

What I have left:

1 corydora
3 glowlight tetras (new)
1 neon tetra
1 dwarf Pearl Gourami
1 hatchetfish
1 sailfin pleco

All the deets:

55 gallon tank. Lots of live plants all thriving. I moved 3 years ago which messed up my nitrogen cycle but after a few months it re-stabilized and I haven’t had a single problem in about 2.5 years. Probably lost two fish in that time, I reckon from old age. The tank has been super low maintenance (all I have to do is water changes and it thrives). I have a large Cannister filter. Keep the tank around 76. I treat new water with SeaChem Prime. Automatic lights with 24 hour daylight cycle (sunset-daylight- evening light- light off overnight)..

On February 22nd, I decided on a whim to go grab some new tetras and plants as I hadn’t added anything new in months. I got 8 glowlight tetras and 3 new plants (PetSmart). I did NOT quarantine these fish (which I’m severely regretting now) and just added them to the tank.

All seemed fine and all fish looked healthy and happy for about a week. Then I noticed one of my corydoras had died. I thought it was a one off. Then I noticed another had died. At this point I was suspicious and gave all my fish a visual exam. Everyone looked happy and healthy. All active, no emancipation or bloat, no faded colors, no skin lesions, nothing. I kind of shook it off and assumed everything was fine. Didn’t pay close attention to the fish this week tbh because I’ve been busy due to our new puppy (just dropping in their food in late at night when it’s too dark to see them). Tonight I finally sat down and did an inventory since I noticed it was looking sparse and was horrified to realize over half of my tank inhabitants have died since last week (when I did my last visual inventory after the two corydora deaths).

I am absolutely devastated as many of the fish who died have been with me for over 3 years. All of the remaining fish look totally fine, but so did the others before they died.

I tested all my levels (see results below) and nothing in there can explain the sudden mass deaths. From Googling it seems like it could be environmental poisoning, but I have no clue how because we don’t use sprays, scented candles, air fresheners, and nothing has changed in our home or their tank other than the new plants and the new fish. I am a fanatic about making sure no soap or detergents or other chemicals go anywhere near my tank. But how could aquarium plants or new fish cause mass sudden deaths either?? I know new fish can bring in lots of diseases but I would think I’d see symptoms rather than fish just dying out of the blue??

Unfortunately I wasn’t able to see their bodies as the resident pesty snails devoured their poor little bodies before I even noticed they were gone. I did look over the two dead cory cats when I pulled them out but didn’t see anything out of the ordinary.


Tested with API Liquid Test Kit (pics attached):

PH: 7
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: Elevated but I think in the 40-80ppm range (see pic), but I honestly think that’s because so many fish have died in the last week, and also the fact I’m sure lots of food was left uneaten this week due to a lower # of living tank inhabitants. I’m also about two weeks overdue on a water change. I do 15-20% water changes (with minor gravel vacuuming) pretty consistently every 1-2 weeks but again, with new puppy was busy and put it off. Even if nitrates were the problem, they wouldn’t be causing sudden symptomless mass deaths like this would they?

My current plan is 25% water changes every few days until my nitrates are back down (don’t want to lower too quickly and cause nitrate shock), but I have NO clue what else to do Do I remove the plants I added (see pics- all were advertised as aquarium plants)? Do I humanely euthanize the 3 remaining glowlights despite them appearing healthy in case they were the source?? I have no clue what the problem is (new plants? New fish? Some kind of environment poisoning? Should I treat for bacteria? Parasites?) so have no clue how to treat it, and I am TERRIFIED I’m going to lose my whole tank. My heart fish, my sailfin pleco, is thankfully still alive and appearing to be doing well, but I will be absolutely devastated if I lose her and all my other fishy friends on top of the 15 I have already lost. I feel horrible that I haven’t been paying close attention to them this week because of the new puppy and wish I had noticed and intervened sooner.

I would be so grateful for any advice.

Its either one of the two reasons.....

1. The new plants you brought back might carry protozoa

2. The ph is high for neons, some strains of corydoras and some of you other tetras

For instance i have 2 strains of corydoras the bronze albino and peppered corys
In my pearl gourami tank where the ph is 7 the 4 out of 12 peppered corys died and two were sick the rest gulping air at the surface but the bronze corys all 6 were doing fantastic so i remove the survivng peppered corys in my angel tank where the ph 6.5 they are now thriving

 

If the plants were tube plants, my money is on the petsmart fish carrying something. I stopped buying their fish without quarantining them. I have had it happen more than once. I would treat and deworm. Honestly If from petsmart type places I will do the Aquarium coop quarantine med treatment. Other places I monitor in quarantine only. It is based of repeated experiences and things employees have said about certain suppliers.
So sorry

 

The first 2 new plants aren’t aquatic, so they will die off eventually. My guess is the new stock introduced something to the tank.

For the nitrates, 50% changes will be more effective than 25%.

 

Adding fish from a big box store without quarantine (and introducing what seems to be a strain of opportunistic bacteria, which is the main reason for sudden deaths after introduction of new fish) and subpar maintenance are a completely sufficient explanation.

You can't really do much. Push the waterchanges to 50% weekly, stop vacuuming (because you remove beneficial microorganisms) and wait it out. The fish are all now confronted with whatever the new fish introduced and their immune systems will have to deal with that. Unless it turns out symptoms after all that point to a treatable parasitosis or disease, do NOT use meds. They may impact immune responses negatively ending in opening a door for whatever was introduced.

Also: Plants do not transmit fish diseases and very rarely parasites.

For the future quarantine and get fish and plants from reputable sources.

 

Seems like overfeeding. Feed a lot less, there shouldn't be any leftover food in the tank.

It takes a lot longer to overwhelm a 55g tank but it will eventually happen if excess food keeps getting added to it.

Cant really rule out disease either, but I'd say unlikely due to the low density of the tank