Sick Danios and Tetras

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Aquarium Advice Activist
Apr 20, 2022
Near Reno, NV, USA
On 7/13 I added 2 GloFish tetras and a GloFish danio to my 20 gallon high tank which had 6 zebra danios in it I believe. I had inherited the GloFish from one of my friend who had housed them in a 3 gallon tank. Right before she gave me the GloFish, one if the danios died. That’s right. In her 3 gallon she had 2 GloFish tetras and 2 GloFish danios. So I took in the 3 remaining fish.

There was no good way to acclimate them as my friend had brought all three in a Tupperware container. I tried to drip acclimate them a little and then netted them into my 20 gallon.

A few days later, on 7/22one of my zebra danios died. Then, on 7/31 another one died. Before they had died, they were just always swimming in the same spot. Not moving forward or up, just staying in place. They also looked pale and skinny. I haven’t noticed any aggressive behavior. If anything, the danios bullied the tetras more that the tetras ever bothered them.

So fast a month later. I had just been on a month long vacation and sat up an automatic feeder for the fish and someone has coming to top off the tank. The day I get home, I do a water change. A few days later, one of the tetras is lookin a bit pointy. I let her be because she still comes up for food. I decide that maybe she is stressed because she only has a school of 2. I go down to the LFS and pick her up two more GloFish tetras. When I return home she is laying on her side on the gravel. I let her be, thinking that maybe she will perk up when I release the new tetras into the tank. She doesn’t. And when she becomes so weak she starts getting stuck to the filter intake, we scoop her out and put her in our mini quarantine hospital tank. She died the next day.

So now here we are. One of the new tetras I got is exhibiting the same symptoms that lead the first tetra to her death. The GloFish danio has also started to become lumpy so we quarantined him.

It is worth noting that my tank STILL grows diatoms (brown algae). One of my friends thinks that my fish are just becoming constipated but I’m not sure. The tetras don’t seem to forage around or “peck” on decorations/gravel.

Any ideas on what might be causing this?

1~What type of fish is afflicted? In addition, please describe what is wrong with the fish to the best of your ability (i.e. cotton like growth, bloated, etc.). Currently: 1 GloFish Tetra and 1 GloFish Danios. In the past: 2 Zebra Danios and 1 GloFish Tetra. (These past fish have died) all the afflicted fish look lumpy. (Pictures below) The danios belly areas looked lumpy while the tetras heads looked lumpy/pointy/flat
2~What are your tank parameters (ammonia, nitrites, nitrates, temp, pH)? Please give exact values. Ammonia:0 Nitrite:0 Nitrate:20-40 Ph:8.2 Temp:~79 Gh and Kh:7
3~ How large is the tank? How long has the tank been set up? 20 gallon high. Set up for around 7 months
4~What type of filtration are you using? Please give the name and number (i.e. Fluval 304) and amount of gph if known. Marineland biowheel 200
5~How many fish are in the tank? What kinds of fish are they and what are their current sizes? 3 GloFish tetras, 4 zebra danios, 1 GloFish danio. Everybody looks pretty fully grown.
6~When is the last time you did a water change and vacuum the gravel? How often do you do this? How much water do you remove at a time? I vacuum once about every 2-3 weeks. I remove about 15-20% of the water.
7~How long have you had the fish? If the fish is new, how did you acclimate it/them? See in above writing
8~Have you added anything new to the tank--decor, new dechlorinator, new substrate, etc.? Nope!
9~What kind of food have you been feeding your fish, have you changed their diet recently? I have been consistently feeding them aqueon tropical fish flakes and dried bloodworms once a week as a treat.


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Fed the quarantined danio some blanched peas in case it is constipation. Attempted to feed the un-isolated tetra peas as well but the other fish are just too fast!
What does the fish's poop look like?
How long did it take the danio to get skinny like this?
How long did it take the tetra to get the sunken area above the head?

If the fish are eating well and their poop is normal coloured (not white), then the danio probably has intestinal worms and or gill flukes. The tetra could also have worms and or gill flukes. These parasites suck the blood out of the fish and the fish waste away.

If the fish became skinny quickly (within a week or two) and are doing stringy white poop, they have an internal protozoan infection that is killing them.

Fish don't get constipated. It's a terrestrial animal thing cause by lack of fluid and fibre. Fish live in water so it can't happen. They can get blockages in their intestine, but this causes them to swell up and die.


You can try feeding them more often (3-5 times a day) with a variety of dry, frozen and live foods. The extra food can help them produce more blood and live longer with worms and gill flukes.

If you feed them more often, do big (50-75%) water changes and gravel clean the substrate every few days to keep the tank cleaner.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.


What to do if your fish has Stringy White Poop.

Fish do a stringy white poop for several reasons.
1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be cured because massive internal organ failure has already occurred.


2) Internal Protozoan Infections cause the fish to lose weight rapidly (over a week or two), fish continues to eat and swim around but not as much as normal, does stringy white poop. If not treated the fish dies a week or so after these symptoms appear. Metronidazole normally works well for this.

There is a medication (API General Cure) that contains Praziquantel and Metronidazole.
It's interesting that API and the Californian government have listed Metronidazole as a carcinogen. That's a concern considering it was widely used to treat intestinal infections in people.

Anyway, if you use this or any medication, handle with care, don't ingest or inhale the medication, and wash hands with soapy water after treating the fish or working in the tank.


3) Intestinal Worms like tapeworm and threadworms cause the fish to lose weight, continue eating and swimming normally, and do a stringy white poop. Fish can do this for months and not be too badly affected. In some cases, fish with a bad worm infestation will actually gain weight and get fat and look like a pregnant guppy. This is due to the huge number of worms inside the fish.

Livebearers like guppies, mollies, swordtails & platies are regularly infected with gill flukes and intestinal worms. If the fish are still eating well, then worms is the most likely cause.

You can use Praziquantel to treat tapeworm and gill flukes. And use Levamisole to treat thread/ round worms. If you can't find these medications, look for Flubendazole, which treats both lots of worms.

In the UK look for:
eSHa gdex contains praziquantel that treats tapeworm and gill flukes.
eSHa-ndx contains levamisole and treats thread/ round worms.
NT Labs Anti-fluke and Wormer contains flubendazole.
Kusuri wormer plus (contains flubendazole) - sold mainly for discus, comes as a powder which is quite hard to dose in smaller tanks
Sera nematol (contains emamectin)

Remove carbon from filters before treatment and increase aeration/ surface turbulence to maximise oxygen levels in the water.

You treat the fish once a week for 4 weeks. The first treatment will kill any worms in the fish. The second, third and forth treatments kill any baby worms that hatch from eggs inside the fish's digestive tract.

Treat every fish tank in the house at the same time to prevent cross contamination.

You do a 75% water change and complete gravel clean 24-48 hours after treatment. Clean the filter 24 hours after treatment too.

Do not use the 2 medications together. If you want to treat both medications in a short space of time, use Praziquantel on day one. Do a 75% water change and gravel clean the substrate on day 2 & 3. Treat the tank with Levamisole on day 4 and do a 75% water change and gravel clean on day 5, 6 & 7 and then start with Praziquantel again on day 8.

The water changes will remove most of the medication so you don't overdose the fish the next time you treat them. The gravel cleaning will suck out any worms and eggs that have been expelled by the fish. Repeating the treatment for 3-4 doses at weekly intervals will kill any worms that hatch from eggs. At the end of the treatment you will have healthier fish. :)
The Danio has started showing symptoms fairly recently. Maybe within the last few days. The tetras has been showing symptoms the day after she was released into my tank. She did have some trouble at the store. She got a little stuck in the net as they were trying to transfer her to the bag.

The previous tetra who died was showing symptoms for over a month though. However, the previous danios only showed symptoms for a few weeks I believe.

I have not seen any of my fish poop (except my female platt who nice healthy brown poops).

As for the medications, will it harm invertebrates? I have a few snails.
Also forget to mention earlier that when the first tetra died, it looked like a few scales were missing.
Scales can fall off or other fish bite them off. If there was a red patch where the scales came off, that is usually an ulcer caused by bacteria.

Some of the medications (mainly Flubendazole) kill snails. Praziquantel and Levamisole don't affect snails. Levamisole would be the first deworming medication to try, but if the fish have lost weight fast, then metronidazole might be a better choice. Rapid weight loss is usually an internal protozoan infection.


If a fish gets caught in the net and the shop has trouble getting it out, tell them to put it back in the tank.

Don't add anymore fish or anything to the tank for at least a month after we work out what is going on.
A bit of excess mucous on it but it's coloured and not an issue caused by an internal protozoan or bacterial infection. It could still have worms or another issue. Can you post a 1 minute video of the fish swimming. It might give us more information.

Upload video to YouTube, then copy & paste the link here.

Post pictures of all the fish you have in the tank. They might show something too.
Sorry, it has been a crazy few days. The pink Danio has died. I bought API general cure and am planning to dose it later today.
The pink tetra died. The other newish tetra, the yellow one, looks like his head is starting to become pointy and his gills look red. I dosed api general cure a few weeks ago. Should I redose it? Should I add stress coat? Salt?


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How often do you feed them?
What are you feeding them?

I would add some salt and if there's no improvement after 2 weeks, try Levamisole or Flubendazole.



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 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 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.

If 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 feed them once a day at 5 pm. They used to get aqueaon tropical flakes but I recently (about a week ago) swapped them over to omega one freshwater flakes. They also get freeze dried bloodworms once a week on Wednesday. Thanks for the salt reply. I will definitely try it!
I would feed them 2-3 times a day and see what happens over the next week.

It could be intestinal worms (thread/ round worms not tapeworm). The praziquantel should have delt with any tapeworm they might have had.

It could be an internal protozoan infection.

It could be a microsporidian infection. Salt treats this, which is why I suggested it. Even if it isn't this, the salt won't harm the fish and will rule this out as the cause. Use salt for 2 weeks.
You don't have to use aquarium salt. If you find non iodised salt at the supermarket, you can use that. You can also use swimming pool salt or rock salt, it's the same stuff as aquarium salt but sold in bigger bags.
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