If you have something leaching, the water at the very bottom of the tank might be get heavy with minerals. Then when you stir up the substrate at the bottom the heavy water gets mixed with the rest, and then the bottom fish might experience osmotic shock.
This all depends on if something is leaching. Some substrates that are specifically for planted tanks can leach. If you are using plant tabs, they can leach also. The lower the pH, the more things will leach.
Right now, all we can really do is guess what's wrong. We can probably figure out most potential problems if we have more complete details.
It be nice to have a full description of the aquarium with all the specifics.
We need to know all the equipment for the tank
What heaters are used, where are they located?
What filters are used, what is their flow rating, how are they set-up?
What lighting is used?
What is the duration of the lighting?
What is the pH at night when the lights are turned off? What is the pH in the morning when the lights are turned on? (This will help us figure out if CO2 levels are getting toxic)
How much surface agitation is there?
Is an air pump or air stone used?
Are fish ever gasping for oxygen in the morning when the lights are turned on?
Do you unplug everything during water changes?
Is any equipment damaged? (Need to examine all the equipment, this means pulling it out of the tank and inspecting it carefully)
Have you ever felt an electrical shock when putting hands in the water?
Is the tank and all equipment electrically grounded?
What fish do you have in this tank, past and present? (Be specific, scientific names please, kuhli loach is a common name for many types of pangio species)
http://www.loaches.com/species-index/species-index
How many fish do you have?
What other inhabitants are in the tank, past and present? (shrimp, snails, ect)
How big is the tank? (Length, Width, Height)
What is the volume of the tank?
How much water is actually in the tank?
What kind of hood is on the tank?
How much water evaporates each week?
What substrate is used?
What decorations, caves, driftwood, rocks are in the tank?
Caves can trap toxins, is there any water flowing in the loach caves?
What plants/algae do you have?
How many plants/algae you have?
What fertilizers are used ?
We need all the water parameters tested accurately.
Temperature, KH, GH, Nitrates, Nitrites, Ammonia, and TDS.
Where you are located? Maybe we can get a water report from the water company.
Need to know your acclimation and quarantine procedures for all the fish.
Dates when the fish were added?
What medications were used and when?
Pictures of everything would help greatly, especially the fish and tank.