For any sick fish, I recommend ruling out environmental problems first as a cause of the signs you are seeing. Built up levels of ammonia and nitrite, in particular, can poison your fish and make them sick. You may see signs such as loss of appetite, lying on side or on the bottom, floating, skin ulceration, discoloration of skin or fins (white patches, reddening of fins or fin bases), eye problems (swelling, cloudiness), tail fraying, lethargy, piping at surface, etc. It is vital to do water testing to know if the water quality is good or if it needs to be addressed.
I would start right away by testing the water for ammonia and nitrite using an aquarium test kit. If you have the ability to also test for pH, nitrate, KH and GH that would also be recommended. If the values are out of range, e.g. ammonia or nitrite are above 0 ppm or nitrate is above 20 ppm, then perform a 30% water change with water that has been treated with a water conditioner to remove chlorine, chloramines and detoxifies ammonia. Also important to check that the water temperature and check that the filter is working properly and exchanging and aerating the water properly.
Some pet stores will do free water testing but that isn't convenient especially if you need to do repeated testing. Home options for water quality testing in the UK include Interpet's Easy Test Complete Aquarium Dip Test Kit or King British's 6 in 1 Water Test Strips for around £12-15 per bottle. The strips test for ammonia, nitrite, nitrate, pH and water hardness (KH and GH), so these would be your best value.
If ammonia, nitrite or nitrate values are elevated, then I would recommend a regimen of water quality testing and, if needed, water changes using water that has been treated with a water conditioner to remove chlorine and chloramines.
-If ammonia or nitrites are not zero but in the 0-0.25ppm, then perform a 25-30% water change. Recheck values daily (in the AM) and water change if values are still 0.25ppm.
-If ammonia or nitrites are 0.50ppm or higher, then perform a 25-30% water change in the AM and another water change in the PM. Recheck values daily (in the AM) and water change if values are still 0.50ppm, continue with twice daily water changes. If the values have dropped to 0.25ppm, once daily water change is ok.
-Once AM readings are 0 ppm for both ammonia and nitrite, you can back off the frequency of water quality testing and water changes to every 3 days.
-Once AM readings remain 0 ppm for two consecutive tests (6 days), then you can back off water changes to weekly and testing to twice monthly.
To help boost the development of your biological filtration, you can purchase and use a commercial source of beneficial bacteria. You add it initially based on the volume of the tank and then with each subsequent water change (based on the volume of the water change). With any of these products, make sure they are well within date, that they are stored in a dark and cool location and that you shake the bottle well before dispensing.
Commercial sources of beneficial bacteria
Dr. Tim’s Aquatics ONE and ONLY Live Nitrifying Bacteria
Fluval Cycle Biological Enhancer
Microbe-Life Aqua Balance
MarineLand Bio-Spira Freshwater Bacteria (FW Only)
API Quick Start or Stress Zyme
Tetra SafeStart Plus (FW Only) or FilterActive
Depending upon your tank size, once you get water quality back under control, you should institute a regular schedule of water changes and tank maintenance. If the tank is less than 20 gallons (75 litres), then weekly 25% water changes are recommended. If your tank is larger and the ammonia and nitrite tests remain 0 ppm after 2 weeks, you could do water changes every 2 weeks.
If using a sponge filter, it should be rinsed out weekly, if using a power filter then media should be cleaned every 2-4 weeks depending upon the size and the amount of fish in the tank. And gravel should be siphoned once monthly to remove accumulated debris. This can be done with a manual aquarium Gravel Vacuum in concert with regular water changes since it removes water in addition to waste from the gravel. I don't recommend doing all the tank maintenance at the same time, to avoid disrupting the biological filtration (good bacterial biofilms) too much. Siphon 1/2 the gravel one week and the other half in a week or two.