Under normal conditions I would 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.
Filters should be cleaned at least once a month and every 2 weeks is better. However, don't clean new filters until they are at least 6 weeks old or you can wash out the filter bacteria and mess up the cycling process. Filter media should be washed in a bucket of tank water and re-used.
You can clean your filter on the same day you do a water change. It is fine to do and most people do it.
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Due to the fish's skinny condition, I would feed him 3-5 times per day, and do a 75% water change and gravel clean the substrate every day for at least a couple of weeks (preferably one month). Post another picture after 2 weeks so we can check his condition. Chances are you will need to keep feeding him 3-5 times a day for at least a month before he will look normal again.
One of the best foods for conditioning fish is raw or cooked prawn/ shrimp. You can buy these from a fishing store (in the bait freezer), or from a supermarket. Buy them frozen and keep them in the freezer. Each day take out one prawn and remove the head, shell and gut (long thin tube in body), and throw these bits away. Use a pair of scissors to cut the remaining prawn tail into small bite size pieces and offer 1 or 2 bits at a time. Let the fish eat as much as possible and then stop feeding and remove uneaten food. The left over prawn can be put in the fridge and used a few hours later. Use a new prawn each day.
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You can buy a variety of frozen foods from most pet shops and some of the more popular choices include brineshrimp, bloodworms, marine mix. These are kept in the freezer and you take out one block and defrost it in a small bowl. Then offer a few bloodworms or brineshrimp and let him eat them. When he has eaten them, offer a few more. Feed until he is full and remove uneaten food.
Some brands of frozen bloodworm have been known to cause problems for fish and some of the members here try to avoid using them. The biggest issue with frozen or live bloodworms is the head. It is very hard and cannot be digested. If a small fish eats too many bloodworms, the heads sometimes block up the digestive tract and the fish can develop an internal infection and die.
You can use a pr of scissors to cut the head off the bloodworms and discard the head. The rest of the bloodworms body is easily digested. The head is the smooth round bit at one end of the bloodworm. If you have any concerns about them, don't bother about frozen bloodworms.
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Marine mix is made up with prawn, fish, squid and sometimes plant matter (spinach or algae). It can be defrosted and cut up into small pieces with a pr of scissors. Feed until the fish is full and then remove uneaten food.
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If you can find live mozzie larvae or live brineshrimp, he should eat them. Mozzie larvae are regularly found in buckets of water outside in the garden. They are the little black wrigglers you sometimes see on the surface of the water. Use a fine mesh fish net to scoop them out, rinse them under tap water and then add them to the tank. Don't add too many because any that don't get eaten will hatch and turn into adult mosquitoes.
Live brineshrimp live in salt water and you can buy small bags of them from most pet shops. Pour them into a bowl and put them in the fridge. Use a teaspoon or turkey baster to take some of the live brineshrimp and put them in his tank. Let him eat them all before offering more. The brineshrimp will swim around for an hour or so but eventually die in freshwater.
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Offer him dry foods as well. Feed him lots of variety to maximise the nutrition he gets.
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Do big daily water changes to keep the water clean. Try to do the water change a few hours after feeding him. If you do a water change straight after feeding, some fish will regurgitate the food.
Make sure you don't have any creams, perfumes, residue from anti-bacterial soaps, grease or anything else on your hands when feeding him and working in his tank.