Hi and welcome to the forum
Is the fish still eating?
What does the fish's poop look like?
If you can post a 1 minute video of the fish it might offer a bit more information. If you use a phone to film the fish, hold the phone horizontally so the footage fills the entire screen.
You can upload videos to YouTube, then copy & paste the link here.
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The cichlids might be attacking the fish or it could be a water quality issue.
If you do bigger water changes more often, you will have lower nitrate levels. I recommend doing a 75% water change and gravel cleaning the substrate once a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If you have a water quality issue (ammonia, nitrite, nitrate), or you think there is a problem with the fish, do a 75% water change and gravel clean the substrate every day until the ammonia, nitrite & nitrate levels are on 0ppm, or until the issue with the fish is identified.
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What sort of filter do you have?
How often do you clean the filter and how do you clean it?
Power filters should be cleaned once a month. 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.