The fish in the pictures do not appear to have any external diseases, which would suggest a possible water quality issue.
As others have said, if in doubt, water change it out. A 50-75% water change won't hurt the fish and can sometimes fix the problem if it's related to something bad in the water. As a general rule, when fish are unwell, the first thing I recommend is to do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes reduce any disease organisms in the water and help to fix any water quality issues by diluting ammonia, nitrite, nitrate or any other chemicals or foreign substance in the water.
Just make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Are you using a dechlorinator (something to neutralise chlorine/ chloramine) in the tap water?
Contact your water supply company (by telephone or website) and ask them if they add chlorine or chloramine to the drinking water.
What sort of filter is on the aquarium?
How often and how do you clean the filter?
Does the filter have any pads/ cartridges in it?
Some filter pads/ cartridges have Ammogon/ Zeolite in and this removes ammonia from the water. If there is no ammonia in the water, the filter bacteria won't develop and you can have 0 ammonia, 0 nitrite, 0 nitrate when you test the water. Normally however, if you have ammonia in the water, it is turned into nitrite and then nitrate by good bacteria, and you get rid of nitrates by doing water changes.
Ammogon/ Zeolite is a white granulated substance. If your filter contains a filter pad with white granules in, and the filter manufacturer recommends you replace the filter pads once a month, then you probably have Ammogon in the filter pad. If this is the case, you should throw the filter pad away and replace it with a piece of sponge. Then keep the feeding down for the next month and do water changes any day the ammonia or nitrite reading go above 0ppm, or the nitrate goes above 20ppm.
Don't try to adjust the pH of the water. It is a losing battle that will cost you money and drive you nuts in the process. A stable pH is much better for the fish than one that fluctuates. And if your water has a high KH
(carbonate hardness), you can add all the buffers you want to drop the pH, and it won't change.
You can normally find out the GH
(general hardness), KH
(carbonate hardness) and pH from your water supply company via their website or phoning them. Find out what the test results are measured in too because they can be measured in different ways and each has different meanings/ results.
Do you have any creams, perfume, hand sanitiser grease or anything else on your hands when you work in the aquarium or feed the fish?
These can sometimes wash off into the aquarium and even in low doses can poison fish.
Do you have buckets specifically for the fish tank?
If not, buy a couple of new buckets and use a permanent marker to write "FISH ONLY" on the side of the buckets. Use those buckets for the fish tank and nothing else.
Make sure any hoses or nets for the fish tank are kept away from chemicals and don't let people use them for anything but the aquarium.
Don't use bug spray, smoke, paint, or do anything in the room that creates fumes because these can get into the water and poison fish.
Make sure the filter is run continuously 24/7.
Have some aeration/ surface turbulence in the water to help keep oxygen levels high.