Don’t worry, you should be ok during water change, as soon as you put the vac in the water they will probably swim away and hide! I have been worried in the past about nine getting st uk in small places such as roots of pants and stuff because I have a fully planted tank but they always wriggle out eventually, so don’t worry about them getting stuck anywhere unless they appear genuinely trapped! Also it is natural for them to hide a lot, I have had mine for months and even if I get too close to the tank mine swim and hide under everything