I'm thinking it was a reaction to the major water change. The loaches have less protection from environental changes than the other fish. Say you've got 40 gallons of water taking into account some displacement for gravel and you're changing out 20% every 14 days, you're only taking out about 8 gallons of water and then again not doing it again for 14 days. You've probably got a pretty good buildup of dissolved organics in that water since you're really not changing enough to dilute and refresh. Then you replaced about 60% of that all at one time. It may have been too drastic a change to them and their skin. Large water changes are not harmful to fish if it's what they are used to. I do about an 80% water change weekly in all my tanks, and when my fry are very young, they're getting 80% changes sometimes every day to remove uneaten food. So there's no significant buildup of anything else in the water to make those changes hard on the fish. You might want to look for the other loaches. If they have gone somewhere and died, you're going to get a nasty ammonia spike which is going to be bad for the other tankmates.