Mystery Deaths - Need Ideas

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Aquarium Advice Newbie
Jun 12, 2023
Stocking at time of catastrophe: 7 ember tetra, about 12 amano shrimp, 10 rummynose tetra.

Stocking prior to catastrophe: 5 ember tetra, about 12 amano shrimp, 3 rummynose tetra.

I purchased 7 rummynose tetra and 2 ember tetras to add to existing schools. (I realize that’s a lot of bioload addition at once, but I was prepared for extra water changes and have been looking for rummynose tetra for about 3 years to replenish my school and ever since supply chain issues, they’d been impossible to find. I drove 2 hours to a highly recommended fish store to buy them) I dosed a small dose of PimaFix and MelaFix as a preventative, which I’ve done with new fish for years. New rummies had colored up nicely and all seemed well after a couple of days. Soon, 2 of my new fish looked a little lethargic and pale, but I chalked it up to transition. The next morning, I walked in to find every fish dead. I tested water parameters frantically amidst the tears and removing them. I was sure I was about to encounter some sort of water parameter problem, but I did not and also found through the major cleaning not a single dead shrimp and every single one of them fine. There was not a single sign of illness on any of the fish after first inspection and I even fridged them and further inspected later after I had myself more emotionally under control.

Not only did the new ones die, but also my 5 existing ember tetra (had them 2 years) and my 3 existing rummynose. (2 of them I’d had 7 years and 1 of them 9 years. Other than cataracts, even my elderly rummies were in good health. I realize that’s past the normal listed lifespan for captivity, but I’ve always been good with tetra) I’ve been keeping fish for over 20 years and have never seen anything like these deaths.

I called the fish store to ask if they’d had any problems with the rummynose or embers they had in stock. They said another client had called them about a die off after purchasing. The lady told me they “should be free of disease because we dosed them up with doxycycline before selling them” Has anyone heard of fish stores doing that? She thinks since my inverts are fine it was some sort of bacterial disease that only affects fish. Has anyone heard of anything like this happening?

I feel absolutely crushed and after 20 years of this hobby bringing me so much joy, I’m thinking about throwing in the towel as I can’t even write this without being a blubbering mess. Of course I’m sad to lose the new fish, but I’m devastated over my rummies that I’d had for 7 and 9 years. (The only fish I’ve lost in the past few years has been rummies from that school dying of old age over the course of several years… it was once a school of 12 rummies)

I feel like I have to get to the bottom of this. Does anyone see anything in this that could have caused such rapid death with no outward signs?
Tank: 37gallon freshwater, heavily planted, PH 6.5-6.6, Ammonia 0, Nitrite 0, Nitrate 5ppm, Temperature 79, filtration 2 Aquaclear HOB with sponges, 2 airstones on pumps. Prime as my conditioner, Flourish line of ferts including comprehensive, excel, trace, iron, and potassium. Weekly 20% water change schedule including gravel vac each week, the 4 filter sponges are squeezed and swished in bucket of tank water with an alternating schedule so I'm only swishing 2 at a time to maintain biofilter, 4 sheets water polishing floss changed out weekly (2 per HOB with the sponges) setup well cycled and established for 10 years. Food is a rotation of freeze dried brine shrimp, micro pellets, and omnivore flake.

I’m sorry for the long post, but I wanted to get all relevant info in. (I performed a 50% water change after the catastrophe and several smaller ones over the past week out of fear for my shrimp. All shrimp are doing well and plants unaffected)

Any suggestions or help would be wildly appreciated.

(Yes, I should have QT and I don't know that I'll ever forgive myself for that seeing as I lost my old rummies, but I can't change that... what I need now is to understand... and make an informed decision if I even want fish again as I'm still reeling from finding all my fish dead a week ago... I don't know if I'm going to be able to shake that.)

Thank you to anyone who made it through my long post! I attached a tank pic for an idea of setup we're talking about.


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If your fish were fine before getting these new fish, and everything died shortly afterwards, and you arent attributing it to water quality, the balance of probability is that you introduced some pathogen with the new fish. Especially as they had another case from a different customer.

Doxycycline is a broad spectrum antibiotic, but its not going to treat every disease. Colin_T and Andy Sagar have both worked in the aquatic pet trade so will know more than me about what's common practice in fish stores. But generally its not good practice to medicate fish unnecessarily. Only medicate when you have a diagnosed problem and you know the medication is suitable to treat the problem. I would say the same with melafix and pimafix actually.

Just to cover bases.

Are you sure you dechlorinated new water with your water changes?

It is the time of year in the northern hemisphere when water companies often overdose on water treatment, so its possible your usual amount of water conditioner isnt enough. Maybe contact the water company and ask if they did anything to water supply recently.
Thank you so much for taking the time to read and give feedback, Aiken Drum.

I use Seachem Prime as my water conditioner and dose at 2X recommended dosage.

Thank you for the advice regarding pimafix and melafix. If I decide to get fish again at some point, I will refrain from using it.
Thanks again

I included a pic of my 9 year old rummy a few days before the new fish (he did have cataracts from age, but had otherwise been in good health and an excellent eater unless someone sees something I've missed) Also pictured is one of the 7 year old rummies and the embers. (I'm not a photographer or great at pics of fish)


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Hi and welcome to the forum :)

Any pictures of the dead fish?

Bacterial infections usually cause red or yellow patches to appear on the fish. They kill one or two fish first and can then wipe out an entire tank within a few days of first showing symptoms. However, if there were no symptoms and all the fish up and died within 24 hours, it is most likely a poisoning event. However, poisoning normally affects invertebrates (shrimp, snails) too, but not always. Some shrimp and lots of snails can tolerate chemicals better than some fish, especially newly imported delicate fishes like rummynose tetras.

Our rummynose tetras never really had issues with bacterial infections (they can get bacterial infections, just ours never did), but they did have issues for a few months each year if they were put in new tanks or tanks with neutral to slightly alkaline water. When this occurred, the fish would stress out and fade, then die if you look at them. We would turn the tank lights off and leave them for a few weeks to settle and they were fine after that. Other fishes in the tank would not be affected like the rummynose were. So this is unlikely to be the issue because you lost 2 species of tetra and your pH is fine for them. I'm not sure what your GH (general hardness) is but if your pH is low, the GH is normally low too. You can contact your water supply company by phone or website to find out what the GH and KH (carbonate hardness) are, or use a GH/ KH test kit and test it yourself. Most pet shops also do water testing, some charge a small few for doing so, others do it for free.

You can also ask the water company if they have done work on the pipes in the area recently. They usually add more chlorine/ chloramine when they do this. They don't normally tell the public they are adding more chlorine/ chloramine and people do lose fish from this. If you did a water change within 24 hours of this happening, that might be a contributing factor.

We never treated rummynose with anything but some shops do prophylactic treatments of newly imported fish. I prefer to only treat when a fish is sick and not unless they are sick. Clean water, a clean tank, and no stress, normally prevent most illnesses from occurring. Doxycyclene is a tetracycline based antibiotic that kills heaps of different bacteria, including filter bacteria. We normally save tetracycline based medications for guppies or neon tetras with external bacterial infections and the fish were treated in bare tanks.

If the shop was treating the fish for a bacterial infection, they should not have been selling the fish until they had finished treatment and the fish given at least 1 (preferably 2-3) weeks to recover from the treatment. If the shop admits the fish were being treated for a bacterial infection when you bought them, you could be entitled to a replacement/s. You would have to contact the shop about it if you want reimbursement or replacement fish.


You should do bigger water changes before adding more fertiliser to the tank. If the aquarium plants don't use all the fertiliser, it can build up over time and poison the fish. New fish are more likely to die from this than fishes that have been in the water as it builds up over time, but eventually it gets to a level that kills the fish that were in there while it built up. I used to do a75-80% water change each week before adding more fertiliser. This diluted any remaining fertiliser and reduced the chance of accidental overdosing.
Thank you Colin T for your feedback.

I did not take photos of the dead fish.

You bring up a really good point on the fertilizer because I fertilized the day before and I wonder if that was an issue... I will definitely take your advice regarding the large water change/fertilizers in case that was the issue. Thank you.

In consideration of your suggestion of poisoning events, I went back through all my actions with that in mind. I had opened a new package of freeze dried brine shrimp the night before the death. (I threw the package away just in case) My tetra always loved those and tended to really gobble, so I fed them sparingly, which means that's something the tetra got that the shrimp didn't because the tetra would have eaten that up before the shrimp could gain access... the shrimp had cucumber that night... unless there was something on the cucumber that leached into the water... though I had washed, scrubbed, and boiled it. The shrimp ate it thoroughly)

I don't know if this means anything, but since the event my largest Amanos have all molted at once. Normally they don't all molt at once... it's kind of a random trickling of molting in most cases, but all 4 of my biggest females all molted immediately along with most of the others. I inspected the empty exoskeletons and found nothing different than usual and they still look fine as a week has passed since the deaths.

Thanks again.
The cucumber is unlikely to be the issue, especially if you wash it beforehand and the shrimp ate it. Make sure you don't use a perfumed soap when washing fruit and veg because they can leave a residue behind. Don't have hand sanitiser or anything on your hands (moisturising cream, oil, perfume, chillis, etc) that can leach into the water, when working in the tank or feeding fish. I used to wash fruit with soap, rinse well under tap water and then peel the fruit. The fish got the fruit and the peels went into the compost.

Freeze dried foods are normally safe unless the packet has been open for more than a few months. The open packets can let moisture in, which can encourage fungus and that can kill things. If you have dry foods, try to use them up within 1-2 months of opening them. Alternatively, put some in a couple of zip lock bags (one bag inside another) and freeze it while you use the rest.

If most of the shrimp molted at the same time, and this was at or just after the event, it might be related. If there was something in the water that poisoned the fish, it might have caused the shrimp to all shed their exoskeleton at the same time. Unfortunately without seeing the fish, it's hard to provide a more accurate response. It might have been bacterial but that shouldn't have wiped them all out as quickly as it did. My money would still be on poisoning of some sort.

At this stage I would stop adding fertiliser for a week and clean the tank up to dilute anything that might be in it. Then wait a few weeks before adding new fish.


Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.

Do a 50-75% (preferably 75%) water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the shrimp and new fish when you get them. It also removes a lot of the gunk, which can lead to harmful pathogens building up in the water.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
Thank you Colin T for the wonderful advice and checklist for a thorough cleaning.

I also have one piece of driftwood... it's been there for 7 years and thus wouldn't have been the problem, but I want to make sure I decontaminated it well. I took it out and used a new toothbrush over it scrubbing into dechlorinated water in a 5g bucket changing the water repeatedly through the process and soaking in dechlorinated/conditioned water until no signs of flakes of driftwood from scrubbing or tannins remained, but is there any decontamination you would recommend specifically for the driftwood?

I still don't know that I'll get more fish, but for the shrimps' ongoing health I want to make sure it's in tip top shape.

Thanks again.
The easiest way to disinfect things for a freshwater tank are drying it out in the sun or soaking it in salt water for 24 hours. Just fill a container with tap water, add the wood and then add a heap of rock salt, swimming pool salt or any sodium chloride. Pool salt is the cheapest and you can buy it from a hardware or chain store. Add so much salt that it doesn't dissolve, then leave the item in the water for 24 hours. After that remove the item, rinse under tap water and then put in a container of freshwater for an hour. Then rinse again and put in the tank.

The other way to disinfect things for a fish tank are boiling or baking items like rocks and gravel, or bleaching plastics. Driftwood should not be boiled because it causes the wood fibers to break down faster. Putting the wood in the oven at 60-80C for 1 hour will kill anything on it and doesn't damage the fibers as much as boiling. A temperature of 60C will kill most life forms on this planet.
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