What are the tank dimensions (length x width x height)?
I change the cartridge in the filter maybe once every one or two months.
How many cartridges do you have in the filter?
It's preferable not to replace filter pads/ cartridges unless they are falling apart. If you replace the pads, you get rid of the beneficial filter bacteria and this can cause water quality issues.
If you have to replace cartridges, then put a sponge in its place. Sponges last for years and can be cleaned by squeezing them out in a bucket of tank water. The sponge gets re-used and the bucket of dirty water gets poured on the lawn.
You can buy sponges for different brands of filter and use a pair of scissors to cut them to fit.
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The fish is stressed out. The neon devil is bullying it a bit and the fish swimming upright is the fish trying to avoid confrontation.
Can you post a picture of the whole tank showing everything in it?
My guess is there is no anemone for the fish to live in, and the neon damsel and female anemonefish are bullying the smaller male anemonefish. The females normally bully the males to stop them changing sex but the combination of the neon damsel and the female could be too much for the male anemonefish.
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The fish's mouth being open is an issue. This could be a bacterial, fungal or viral infection, possibly brought on by stress.
In a freshwater aquarium, I would suggest doing big daily water changes and adding some salt. Salt won't help with marine fish because they live in salt water. But you could try doing a couple of huge (90%) water changes. Basically drain the tank so there is just enough water for the fish, then refill it with clean sea water. As long as the temperature and salinity are the same, you can do this without an issue.
Do the 90% water change every couple of days for a week or two and see if it helps. If there's no improvement after a couple of big water changes, you might need to treat the tank with a broad spectrum medication like Triple Sulpha.
Adding an anemone might help, and possibly having lots of rock in the tank might help.
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If you are using artificial marine salts to make up the salt water, make the salt water up and aerate it for at least 24 hours before using it. This allows all the salt and minerals to dissolve completely before being added to the tank.