First thing to mention is your earlier thread largely on this issue. That was part of the problem, but not all of it from what we now have here, so I will offer some guidance on the whole issue.
Kuhli loaches is a shoaling/schooling species that needs a decent sized group. I'm afraid that three or four is not a decent sized group. That is a serious issue, because we have clear evidence now that shoaling/schooling species need groups in the 10+ range. Denied more in the group, the fish will be stressed, likely show increased aggression (aggressive species became more aggressive, while peaceful species became aggressive to varying degrees), and even have a latency to feed. The latter illustrates just how serious numbers are, when fish are reluctant to eat just because of their insecurity.
The light we did cover in the other thread, so I need only repeat that there can be trouble with bright light (intense light for plants was mentioned, but no details were given), and turning the tank light on or off in a room that is not well lighted at the time can shock fish.
Water changes do and will invigorate most fish, that is not a problem, far from it. Unless there is a significant difference in GH, pH or temperature between the tank and fresh water. "Significant," not minor or normal.
In cyprinids, especially loaches which belong to this Order, erratic behaviour is a sign of something that is seriously affecting the fish. Fish do jump to escape such conditions, and loaches are well know to jump out at night, so make sure there are no openings that may allow this.