As long as the water chemistry is close, you can safely change any amount of water - as long as the fish have enough water to swim and not freak out. If you were to actually remove all the water (putting the fish in a holding container), and the chemistry of the new water was quite different, then you would just acclimate the fish to the new water in the tank - just as you would when you buy a fish. It's really only a concern for people who NEVER change their water or only do very small water changes. loaches love water changes, so don't be afraid
Botine loaches are known for their quirky behavior. It's somewhat normal to find one laying on it's side. Not to say that it wasn't indicative of a problem, obviously as your fish died. But, it is actually "normal" behavior for them to do so from time to time. Not sure if you have a school or if you intend to get one or not, but they can get pretty aggressive when kept alone or in small groups, and can still harass other fish when in a larger group. Just a heads up.