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Please help! Guppy missing fin and laying on side

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New Member
Mar 9, 2023
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United Kingdom
Please help, your advice is much appreciated! We are fairly new to fish keeping and we have just spotted today our guppy fish who we bought a couple of weeks ago is missing most of his caudal fin and is struggling to swim. We have separated him out using a quickly homemade medi box within our tank with drilled holes in a net but he isn’t looking good. He swims for a few seconds then is laying on his side at the bottom of the box. We put him in the box as he was struggling to swim with the current and kept getting stuck.

Do you think he is going to pass away? Do we need to separate him from the rest of our fish? We don’t have a quarantine tank currently. We have checked the other fish and can’t see signs of fin damage on them. We don’t know if he’s been nipped or caught it on something or has fin rot or another illness.

I feel so sorry for him and would appreciate any help with how we can help him.

Many thanks.
Do you have any pictures or video of the fish? If he's struggling to swim it may be kinder to euthanise him but I don't want to say for certain without seeing how he is. Is he able to eat?
Test the water for ammonia, nitrite, nitrate and pH. Post the results in numbers here.

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.

Post clear pictures and video of the fish so we can check them for diseases.
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 you don't get black bars on either end.
Sorry to hear this, it may be worth doing some precautionary large water changes just in case there is something that caused this guppy to become so unwell, and monitor the remaining fish closely.

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