Any ideas why a fish would do this, it is concerning me as it's just sitting on the bottom most of the day and appears to be flapping its fin a lot to do nothing.
Video of fish
Video of fish
Hi and welcome to the forum 
What sort of fish are you talking about?
How long has the tank been set up for?
What is the ammonia, nitrite, nitrate and pH of the tank water?
Can you post some pictures of the fish and try uploading the video again, I can't view the video?
In the mean time, do a 75% water change and gravel clean the substrate every day until we work out what is going on.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Increase aeration/ surface turbulence to maximise the oxygen levels in the tank.
What sort of fish are you talking about?
How long has the tank been set up for?
What is the ammonia, nitrite, nitrate and pH of the tank water?
Can you post some pictures of the fish and try uploading the video again, I can't view the video?
In the mean time, do a 75% water change and gravel clean the substrate every day until we work out what is going on.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Increase aeration/ surface turbulence to maximise the oxygen levels in the tank.
The video is working from my wifes phone, not sure whats up.
It is most definitely a sex pest male fish
I assumed water parameters but there are 15 other fish that seem fine.
-Fish - MollyHi and welcome to the forum
What sort of fish are you talking about?
How long has the tank been set up for?
What is the ammonia, nitrite, nitrate and pH of the tank water?
Can you post some pictures of the fish and try uploading the video again, I can't view the video?
In the mean time, do a 75% water change and gravel clean the substrate every day until we work out what is going on.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Increase aeration/ surface turbulence to maximise the oxygen levels in the tank.
-Tank has been set up for over a year
-Nitrates etc, no idea
-I change 25% of the water every month after leaving the water for a week to dechlorinate.
-I have a 10 gallon pump running a 4" air stone in a 10 gallon tank with a filter that is separate
i think if you just dechlorinate it, ammonia, nitrites, and Nitrates are still there in the water and are not detoxified, a water conditioner is probably a better idea
@Colin_T is this right or am i hallucinating
A basic dechlorinator like Wardley's Tristart only contains sodium thiosulphate, which neutralises chlorine. It does nothing to ammonia, nitrite or nitrate.
Some brands of dechlorinator neutralise chlorine and bind with free ammonia and nitrite. This type of dechlorinator is normally used when chloramine is in the water source. The dechlorinator breaks the chlorine ammonia bond, neutralises the chlorine, and binds to the ammonia so it is less toxic to the fish. The filter then uses the ammonia.
Without knowing the water quality (ammonia, nitrite, nitrate and pH), and without knowing whether there is chlorine or chloramine in the water source, we can't offer much more info until we see the fish.
If the OP just has chlorine in the tap water, then letting it stand for a few days will allow the chlorine to come out by itself. If they have chloramine in the water, they will need a dechlorinator that deals with chloramine.
Some brands of dechlorinator neutralise chlorine and bind with free ammonia and nitrite. This type of dechlorinator is normally used when chloramine is in the water source. The dechlorinator breaks the chlorine ammonia bond, neutralises the chlorine, and binds to the ammonia so it is less toxic to the fish. The filter then uses the ammonia.
Without knowing the water quality (ammonia, nitrite, nitrate and pH), and without knowing whether there is chlorine or chloramine in the water source, we can't offer much more info until we see the fish.
If the OP just has chlorine in the tap water, then letting it stand for a few days will allow the chlorine to come out by itself. If they have chloramine in the water, they will need a dechlorinator that deals with chloramine.
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