What is the ammonia, nitrite, nitrate and pH of the water (in numbers)?
What sort of filter is on the tank?
How often and how do you clean the filter?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
How long after you got the second batch of Khuli loaches did they start dying?
How much time elapsed between you getting the first and second batch of loaches?
Did you add any other fishes around the time you got the loaches?
Are they eating and if yes, what are they eating?
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It looks like the fish is covered in excess mucous. This is normally caused by something in the water irritating the fish and the fish respond by increasing their mucous production. Fish naturally have a thin layer of mucous over their body to help move through water and to act as a first line of defence against pathogens and chemicals in the water. The more mucous they have the whiter they become.
The other option is a disease but they don't appear to have an external disease that I can see. They do look skinny and if they had just come into the shop shortly before you got them, they might have been suffering from stress after going from fish farm, to exporter, importer, wholesaler, pet shop and then your tank. This could have weakened them and the difference in water chemistry between where they originated and your tank (and everywhere in between), could be enough to kill them.
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BASIC FIRST AID FOR FISH
Test the water for ammonia, nitrite, nitrate and pH. Post the results in numbers.
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 or until the problem is identified. 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.
Post a video of the fish so we can check them for issues that might show up when they move around. Try to get them to swim in the video.
You can upload videos to YouTube, then copy & paste the link here.
If you use a mobile phone to film the fish, hold the phone horizontally (landscape mode) so the footage fills the entire screen and doesn't have black bars on either end.
