Can't tell anything from the pictures but if the fish suddenly bloated up overnight, has stopped eating and did a stringy white poop, then it has an internal infection and will probably be dead tomorrow. If this has happened, then euthanise it.
You can try the following but if the fish have an internal infection, it probably won't help.
Test the water for ammonia, nitrite, nitrate and pH.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
If you are losing fish regularly from the same symptoms, perhaps take one to a fish vet and have them necropsy (animal autopsy) it. You might also be able to get this done at the Department of Agriculture in the Fish Health section (if there's one nearby). Dept of Ag doesn't normally charge for home aquarists but might so call them first and see if they have a Fish Health section and if they have anyone who can help you necropsy the fish.
