In answer to what makes the fish "misty", it is made from opalescent glass. This requires the addition of potash (and sometimes arsenic), into the hot glass which is what produces the "colour" effects.
It will look misty, milky blue with light bouncing off it, but when held up to the light, so that the light comes through the glass, you will find it has an inner "firey glow2 of ambers and reds - much like sunset colours!
How dense the glass is will affect the effect.