Xenia, when they melt, can be EXTREMELY toxic. They pick up and store all sorts of "stuff" from your tank - things that would normally be exported thru water changes. When they die, they release all of this at once, which can easily pollute your tank. Xenia crashed my daughter's nano a couple of years ago.

As for your six line - they can do a disappearing act, like all wrasses. Either into the rock/sand for a few weeks, or over the top. They are amazing jumpers.

Salt really only has to be mixed until the water is clear. I do this ALL THE TIME, without problems. In my daughter's nano, it gets a 50-60% water change about every 3 weeks - everything does just fine in that tank.

BTW, BTAs can be real polluters too - I almost never feed mine, and this helps.