What was the medication that you used a couple of weeks ago?
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If you can lower the airline down and have them closer to the bottom of the tank, it will circulate and aerate the water more effectively. You want the bubbles coming out of the airline as close to the bottom of the tank as you can. If you can find multi-coloured plastic airstones, then put one on the end of the airline to break up the bubbles more. Try not to use normal blue or brown airstones because they put a lot of pressure on the pump and always block up. Do not use wood airstones either because the bubbles are too fine.
You can tie a small fishing sinker to the end of the airline so it doesn't float around.
You can lower the water level down an inch or so and it will increase aeration from the filter.
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Putting a backing on the tank might help reduce stress and make the fish feel more comfortable. You can use anything on the back of the tank. Just tape some newspaper, coloured card, plastic bin liner to the outside of the tank on the back. Try to use something dark so it reduces stress.
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Maybe put a few small caves in the tank (one for each fish) and that might help reduce stress. Spread the caves out across the tank. Make sure you can see inside and around the caves so a fish doesn't die in one and pollute the water. Small plastic pots (for gardening) laying on their side and half buried in the gravel can be used. Have the open part of the pot facing the front of the tank so you can see into them. Make sure the pots are clean and have not been exposed to chemicals like herbicides or pesticides.
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Having a fish die an hour or so after feeding might indicate there is ammonia in the water that is not being picked up by the filters quick enough, and that is poisoning the fish.
If you can find some filter sponge from any brand of filter and add them to your current filter it will provide you with more filtration area and that might help. You can cut up sponges with a pr of scissors.
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In the morning open the curtains or turn the room light on 30 minutes or more before turning the tank light on. This will reduce the stress on the fish and they won't go from a dark tank to a bright tank instantly.
At night turn the room light on and then turn the tank light off. Wait 30 minutes or more before turning the room light out. This allows the fish to settle down for the night instead of going from a brightly lit tank to complete darkness instantly.
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You can add some floating plants like Water Sprite (Ceratopteris thalictroides/ cornuta) and it will make the fish feel more secure and absorb ammonia from the water, which should help. You can make a loop of plastic airline or plastic tubing and tie it to some string and attach the other end of the string to a suction cup on the glass. Then put the floating plants inside the loop of tubing and it should stop them floating about the tank and being sucked into the filter.
Water Sprite can also be planted into the gravel.
If you don't want live plants then add some plastic plants and tie a small fishing sinker to the base of them. Make sure you can see around and into the plants to check for dead fish. You can also let plastic plants float on the surface to provide shade and shelter for the fish. Live plants are better tho because they use nutrients like ammonia.
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I initially said not to raise the temperature, but if the fish have a protozoan infection, you can raise the temperature to 30C (86F) for 2 weeks and that will kill off any protozoans in the water. Increase aeration when you do this to maximise the oxygen in the water.
If the current water temperature is more than 3C lower than 30C, then increase the temperature by 2-3C each day until you get to 30C. Keep the temperature at 30C for 2 weeks and then slowly lower it back down (2-3C each day).
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Possibly look into filtering your tapwater before using it. Set up a couple of big plastic containers (rubbish bin, storage container, etc), fill it with water and add a dechlorinator. Do not use a dechlorinator that binds with ammonia and makes it inert, just use a basic dechlorinator to remove the chlorine.
Add an air operated box filter or power filter with carbon and Ammogon/ Zeolite in and allow it to run on the storage container for a day or two. Check the water for ammonia and when it has none, then use that water for water changes.
Zeolite can be recharged by soaking in salt water for a day.
Carbon should be replaced every couple of batches of water.
This might fix the problem but it might not. Right now we need to try and work out what is going on and by doing these things, we can rule out chemicals in the water and stress in the tank. If the fish continue to die after several weeks of doing this, then there is something in the tank that is causing the problem. It could be something in the substrate or a disease.
If the substrate was bought from a pet shop it should be safe. If there is a protozoan infection in the tank, that will be killed off by increasing the temperature.
