You should only replace filter media/ materials when it starts to break down. If you replace the media, you remove the beneficial filter bacteria and can have ammonia or nitrite readings occur in the tank and these can harm the fish. If you have to replace filter media, only replace part of it and wait 2 months before replacing any more.
You can put sponges in most filters and sponge last for 10+ years. You can buy sponges for different brands of filters and use a pair of scissors to cut them to fit your filter. Sponges get squeezed out in a bucket of tank water. The sponges get re-used while the bucket of dirty water gets poured on the lawn.
Established filters should be cleaned at least once a month.
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The fish looks skinny and sunken in around the back of the head. This could be caused by intestinal worms, gill flukes, cancer, lack of food or bad food.
If the food is fresh it should be ok. However, if the packet of food has been open for more than a couple of months, it might have gone off or have fungus growing on it. It's preferable to use up fish food within one month of opening it.
If you aren't going to use all the food in a month, you can put half in an airtight container and pop it in the freezer until you need it.
You could try offering more variety in food. Frozen foods like bloodworm (you already use), brineshrimp, daphnia, mysis shrimp, raw or cooked prawn/ shrimp are all useful and can be fed each day to try and build the fish up.
You can feed him live brineshrimp, daphnia, mozzie larvae, aphids, small flies and moths (that are free of chemicals and pesticides).
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What does his poop look like?
You could try deworming him and see if that helps. I would treat for round worms first. If he doesn't improve after a month then treat him for tapeworm.
Section 3 in the following link has information about treating intestinal worms in fish.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
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