What do you feed the fish?
How often do you feed the fish?
If you only feed dry food, perhaps add some frozen or live foods to his diet. There are a number of frozen fish foods available from most pet shops and they include things like brineshrimp, daphnia, bloodworms, Mysis shrimp, marine mix (prawn, fish & squid mixed up).
You can also get live brineshrimp and daphnia from most pet shops. And if you have roses in the garden, look for aphids and tap them into a bucket and feed to the fish. He only needs a few and extras can be frozen in a plastic bag or container for use later on.
Mosquito larvae are a good food if you can get them. They can be found in containers of water sitting outside under trees and plants.
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The fish might be getting old.
You could try putting a picture on the back of the tank and add some floating plants like Water Sprite (Ceratopteris thalictroides/ cornuta).
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Clean the filter once a month. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
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You can put a mirror against the side, back or front of the tank for a couple of minutes once a week, or once every couple of weeks. The male will think it's an intruder and get all angry with it and show off. After a minute or two you remove the mirror and the fish thinks he has driven off his competitor and will usually strut around the tank feeling good about chasing away the intruder. Don't do it too often but once a week for a few minutes can break up the usual routine.