Fish dying one by one

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Apr 4, 2024
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My fish (guppies and mollies) have been dying one by one these past few weeks/months. Two of them (mollies) presented the same sypmtoms you can see in the video before dying: swaying from side to side like they have lost their balance and staying near the bottom of the tank. The molly shown in the video (Edit: didn't let me post the video) is still alive though, but I'm guessing it will die today/tonight. The first molly that died like this also had trouble swimming upward and it would start spinning like crazy two days before dying. Two guppies have also died. One of them stayed near the bottom, then a few days later stayed near the top of the tank without moving much and then it died. The first guppy that died presented different symptoms: dropsy and also some lethargy. I'm confused because the symptoms vary fish from fish. I already restarted this tank (29G) all over again on December because it had been neglected, so I don't know if the fish that were already in the tank were carrying a disease. I have one tetra (veteran survivor of a tetra school I had) that has lived in this tank since before starting the tank all over again and it's been pale for about a year now. Lately its gills look red, but not all the time. I don't know if that fish is infected too and maybe responsible of infecting the others, sometimes it quivers in a weird way (these last few days), but it has survived way too long without dying. I thought it would die months ago but it's still active and alive, although looking physically bad. I don't know what to do and these past few days it's gotten worse and fish started dying faster. Should I restart the tank again? Should I use a medicine? I added salt today but I know that won't be enough. I've been doing weekly 20-25% water changes since I restarted the tank. I also use Easy Green, but only 3 mL a week.

The tank doesn't have a heater because I live in a tropical country, I also notice that sometimes after doing a water change fish airgasp for a while. About 2 years ago I had a case of TB in the tank, I got rid of the infected fish and no one else got infected, which made me doubt later if it was actually TB or something else. When I restarted the tank in December I rinsed the gravel a lot, I let the decoration dry for a day or two and rinsed it, I replaced the sponge and cartridge of the filter, rinsed the glass with water, used some new plants from another tank which is doing fine as for now.

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Need pictures and video of the fish. You can upload videos to YouTube, then copy & paste the link here.
If you use a mobile phone to film the fish, hold the phone horizontally (landscape mode) so the footage fills the entire screen and doesn't have black bars on either end.


Shimmying is where fish swim in the same place. They do an exaggerate S but don't really go anywhere. It's common in mollies kept in soft water. Mollies need lots of minerals in the water and the GH (general hardness) should be at least 250ppm.

A temporary treatment for shimmying is adding salt to the tank. Use 2 heaped tablespoons per 20 litres. But you will need to get a Rift Lake water conditioner (mineral salts) to fix the problem long term if you have soft water.

What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).

Depending on what the GH of your water is, will determine what fish you should keep.

Angelfish, discus, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.

Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0.

If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.


Spinning/ spiraling through the water is normally caused by a bacterial, viral or protozoan infection in the brain. There's no cure for the infected fish but you can normally stop it affecting other fish by cleaning the tank up (big water changes, cleaning the gravel and filter) and adding salt (2 heaped tablespoons per 20 litres of water).


Test the water for ammonia, nitrite, nitrate and pH.

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 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
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.

Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.


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 tablespoon per 20 litres (5 gallons) of water.

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.

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