Bacterial Infection Running Rampant Through Tank

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Aquarium Advice Apprentice
Jan 14, 2023
Australia (QLD)
TLDR: I had a fish die in the tank and go unnoticed. Ammonia levels spiked and a bacterial infection has quickly developed and spread throughout my poor stressed fish.

Fish (as of 48 hours ago):
- 6 pepper corys - in tank for 6 months

- 1 albino bristlenose - in tank for 4 months
- 10 betta females - in tank for 1 month (approximately 6 months old, 6 from same spawn, 4 from another. Introduced at the same time, no problems till 48 hours ago)
- 20 cardinals - in tank for 5 days (NOTE: they were not quarantined before being introduced... I know, never a good idea)

Tank Size - 50 gallons / 6 months old
Filter - External canister filter, internal power head filter.

Normal Tank Parameters at 15/01/23 and before:
pH - 7.86

Ammonia - 0.00
Nitrite - 0.00
Nitrate - 0.00
Temperature - 27 celsius / 80.6 farenheit

Tank Parameters Today on 18/01/23
pH - 7.86

Ammonia - between 1.5pm*

Nitrite - 0.00
Nitrate - 0.00
Temperature - 27 celsius / 80.6 farenheit

25% water changes weekly. Last water change done yesterday 17/01/23 which was 100%. Water change before that happened on the 15/01/23. I did not test the levels at the 100% water change. Tested the water levels today before transferring all fish to the hospital tank.

What Happened:

Had an absolutely catastrophic 48 hours. One of my bettas got stuck inside a rock. I had no idea the hole in that rock existed, or that she could fit inside it. Terrible oversight on my part. I had ten bettas, mostly all from the same spawn so it isnt always the easiest to tell them apart.

I did not notice one missing at their nightly feeding time. I did not notice anything amiss until the next morning when I saw the dynamic of the tank had changed dramatically. There was a lot of fighting. As soon as I saw the behavior I started removing fish from the tank and into temporary holding vessels.

I found my poor betta stuck in that rock. She was so badly wedged I couldnt even remove her body to bury her. I found another two dead betta and several dead cardinals who who did not have any obvious wounds. I suspect the cardinals died from ammonia poisoning, but I cannot be sure.

I completely drained the tank, cleaned the substrate and filled it back up with water.

I put the cardinals, plec and corys back in the tank. They all appeared fine physically though clearly stressed.

The healthiest bettas were put in the hospital tank, each separated by dividers. The ones with obvious injuries or sicknesses, I kept in jugs, hanging on the side of the 50 gallon tank. The water would not be filtered but at least it would be warm. Every betta received an epsom salt bath, and a dose of melafix.

I woke up this morning to another dead betta fish, and had to euthanize three others who were refusing to eat, laying on the bottom, pineconing and breathing heavily. They had ich, which I didnt see the day before, and an obvious bacterial infection.

I have gone from ten bettas to three in 48 hours. Of the three I have left, two appear fine. Active and eating. One is not, she has an obvious bacterial infection. I bought Aqua One's 'Broad Spectrum Remedy' today and have dosed the hosptial tank with it, and have been continuing with Melafix.

As if matters couldnt get any worse, my cardinals are displaying this infection now too. They are new, only been in the tank for 5 days, so I cannot speak long term of their health. I have lost more today. Many of them have ich and three have have an obvious bacterial infection. They are now also in the hospital tank.

I do not know what to do about the cory's and plec. They seem okay, but I have them in the hospital tank too just in case. 50 gallon is totally drained (again) and will be deep cleaned before I even consider putting anything back in there.

Remaining Fish:
- 6 pepper corys - in hospital tank as of Today

- 1 albino bristlenose - in hospital tank as of Today

- 3 betta females - in hospital tank as of Yesterday

- 10 cardinals - in hospital tank as of Today

Active Treatment:
Dosed with Aqua One's 'Broad Spectrum Remedy'
Daily recommended dosage of 'Melafix'

NOTE: I am in Australia where fish antibiotics are not easy to access

I am absolutely devastated. Feel like a total fish killing monster. I've dealt with infections before, but never to this scale. Any advice at all would be incredibly appreciated. I suspect I will lose two more cardinals (minimum) overnight as both have stopped schooling and eating. I am considering euthanizing them with clove oil, but ****, where does it stop? I dont want things dying in my hospital tank and spiking the water in there, but I dont have the means to run multiple hospital tanks and separate everyone more than I already have.

Specific Questions:

- Plec and corrys seem okay, but what should I watch out for with them?

- Tetras seem like notoriously sensitive little things, how can I improve their chances of pulling through?

- I feel like have some handle with treating the bettas as they are the fish I have the most experience with, but what do I do with the two relatively unaffected girls? Is it safe to keep them in a tank where other fish are infected and dying? I know they probably have the infection already and just arent actively showing it, but I really want to keep it that way.

-Moving forwards, how do you prevent things like this happening in large planted tanks? I feel horribly guilty that I didnt notice the missing fish sooner, but there was no way of finding her body without pulling the tank apart. What about when something like a tetra dies? How on earth are you supposed to find those little things?

Any pictures of the fish so we can confirm bacterial infections and white spot?

Melafix can be poisonous to labyrinth fishes (Bettas & Gouramis) because it leaves an oily film on the surface of the water. When the labyrinth fish goes to take air from the surface, they get the oil on their gills and labyrinth organ and can suffocate.

Fish naturally have a thin layer of clear mucous over their body and fins. It helps them move through the water and acts as a first line of defense against microscopic organisms and chemicals in the water. If you overdose fish with any chemical (plant fertiliser, medication, etc) they can produce more mucous over their body, which can look like white patches or spots. The excess mucous is a natural reaction to something in the water irritating the fish.

Mixing medications is a quick way to kill fish. Having Melafix in the tank and then adding a broad spectrum medication can severely stress or kill fish. If you overdose either medication, that makes the problem much worse.


I would stop adding things to the tank and do a 75% water change and gravel clean the substrate every day for a week, or until we can properly identify the problem.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Post pictures of any sick fish asap.
I will post pictures later today once I'm back home.
I take it you don't recommend melafix/Betta fix for Bettas? I'll stop using it immediately. Ive heard bad things about it before but it's what I had on hand.
The hospital tank has had at 3/4 water change today.
The tetras have patches of discolouration on them, and a few have white spots that look like ich. I could be misdiagnosing. Hopefully I am
Spoke to an aquarium manager but he wasn't able to offer much advice. Another water change done on the hospital tank. Cory's and Plec look find, considering seperating them as they are at risk of ammonia poisoning if the remaining tetras die through the night or while I am at work again.

Here is a link to the photos. They contain photos of dead cardinals, so not a pretty sight.
A lot of the tetras have this strange bent body. This one is still alive in the photo. I am awaiting moderator approval for the rest of the photos to be posted.
I'm going to go ahead and post the photos using the the forums upload system rather than an Imgur link. Later there should be a post come through (once it is approved) with the link but for now, these are some of the clearer photos I have managed to get.

I swear I can see little white spots on some of the tetras but I cannot for the life of me manage to get a good photo of it. It is mainly on their tails.

Some of the tetras (most have now died) had weirdly bent bodies and were struggling to swim.
Many tetras are missing tails when I find them dead. This could very well be from the other tetras or even the cory. Some of them have had this while alive though.
All look very desaturated and they have patches of discolouration that looks like this: note, not my photo


EDIT: Bettas have stabilized and are looking better, including the one with the patches of discoularation and red eyes (eyes have returned to normal). Corys still fine, as is plec. Tetra's are dying like flies still. I lost half again since my last post.
It's a bit hard to tell anything from the picture but if that is a cardinal tetra and the red line is orange, it has a bacterial infection like neon disease. Antibiotics are the main thing you need but finding them in Australia is like looking for a Thylacine (aka Tassie tiger).

You can try salt with the broad spectrum medication but I don't have a lot of hope for any of the fish that are infected. Use 2 heaped tablespoons of rock salt (non iodised salt) for every 20 litres of water. Keep the salt in there for 2 weeks and run the broad spectrum medication or an antifungal/ antibacterial medication at the same time.

The fish will be fine with the salt at that dose rate and should be ok with the broad spectrum medication but you might need a lower dose of medication for the catfish, which might not treat the bacteria.


Before you treat the tank, work out the volume of water in it.
measure length x width x height in cm.
divide by 1000.
= volume in litres.

When you measure the height, measure from the top of the substrate to the top of the water level.

If you have big rocks or driftwood in the tank, remove these before measuring the height of the water level so you get a more accurate water volume.

You can use a permanent marker to draw a line on the tank at the water level and put down how many litres are in the tank at that level.

Remove carbon from the filter before treating with chemicals or it will adsorb the medication and stop it working.


You should pop down to the pet shop you got the cardinal tetras from and check their fish to see if they have any issues. While there tell them what has happened and they might credit you for the dead fish. But don't add any new fish to the until this has been dealt with and the tank has been fine for at least a month.
Thank you very much for your reply. The aquarium I got them from is a bit on the dodgier side. I dont normally buy from them as they keep their poor betta's in cups but my usual aquarium doesnt stock caridnals. They keep their cardinals and neons in the same tank, though that may be unrelated to my fishes current health problems.

All the tetras are orange so that's good to know that is an indicator for a bacterial infection moving forwards (if I'm ever brave enough to have cardinals again). I dont have much hope for them either. Every other fish has stabalised and they are still dying constantly. I am nervous about keeping them in the same tank as my other in recovery fish because I dont want to spike the water. I have the bettas in a seperate tank and am thinking about moving the corys and plec back into my 50 gallon now that it has been deep cleaned and set up again. It wont be properly cycled yet, but I have taken healthy water and filter material from other tanks to hopefully kick start the process and have a testing kit to keep track. I have a 6 stage reverse osmosis filter that I use for all my fishes water changes, and while I dont like to do in take cycling, I think it is a better option than keeping them in the tetras hospital tank.
I will pop into the aquarium tomorrow. I did post up a bunch of other photos, but I think they are stuck waiting for moderator approval (at least I cant see them on my end). The tetras are one thing, but I am just absolutely heartbroken about the bettas. I raised those girls from fry. Bred them myself. I hadnt planned on having a sorority because I know how badly they can go but when I attempted to give them to an aquarium (for free mind you) they wouldn't take them as they were ugly by betta fish standards. Said there wasn't enough demand for girls, that they wouldnt sell and that they'd end up euthanized. They took my boys, and some of the colorful girls but not these ones.

Any advice for the corys and plec? Should I just keep an eye on them to watch for any infection or should I be preemptively treating with anything?
If you ever get tetras again, check them for pale blue or red lines, which patches anywhere on their body or fins, clamped fins, and look to see if any of the fish are sitting in a corner or near the bottom or surface and not hanging out with the others. These are all signs of sick fish. If one fish in the tank has any of these symptoms, do not get anything from that tank.

If you don't have a quarantine tank, you can use a plastic storage container as a temporary holding tank for any new fish. All new fish should be quarantines for at least 2 (preferably 4) weeks before being added to an established aquarium containing other fish.


The last picture you posted is a neon tetra, the others are cardinal tetras. Neons are more prone to bacterial infections and tend to come in from the suppliers with the infection. Most shops keep neons and cardinals apart for that reason, and because cardinals cost twice the price of neons so they don't want staff selling cardinals as neons.

As for the catfish, I would do a big (75-80%) water change and gravel clean the substrate every day for a week. this will help dilute any bacteria and reduce the chance of the other fish getting it. Then monitor them and hope for the best.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Clean the filter too if it hasn't been done in the last 2 weeks. Again this reduces the number of disease organisms in the water and lowers the risk of the other fish catching it.

You can add some salt to the catfish tank. Salt can help kill some of the bacteria and might help reduce the risk.


You can add rock salt (often sold as aquarium salt), swimming pool salt, or any non iodised salt (sodium chloride) to the aquarium at the dose rate of 2 heaped tablespoons of salt per 20 litres.

Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, Bettas & gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.

The salt will not affect the beneficial filter bacteria, fish, plants, shrimp or snails.

After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.

When you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.

When you first add salt, add the salt to a small bucket of tank water and dissolve the salt. Then slowly pour the salt water into the tank near the filter outlet. Add the salt over a couple of minutes.
I went to the aquarium today. All their Cardinals have ich. The tank is full of dead Cardinals, some appear also to have bacterial infections probably having a weekend immune system.
They won't do anything about it, which is about what I expected. Shouldn't have bought from them as I've had bad experiences in the past but ****, I'm heartbroken and frustrated by the whole experience.
I have deep cleaned my tank, and will be treating everyone with the broad spectrum product which should help with both ich and the bacterial infection the Cardinals have.
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