Hear me out before you judge.... I have an idea when it comes to replacing substrate. Looking to hear your thoughts on if it is a good idea.
Im thinking of removing most of the current substrate, and placing it in a pair of tights(UK) i think you call them pantyhose in the USA, tying the end in a knot. Then putting the new substrate down and placing the bag of old substrate on top. That way the bacteria in the old substrate will still be in the tank for a few weeks. Then after that I can remove the tights filled with old substrate and throw it away.... don't have to worry about old mixing with new (different color). Also no need for the hassle of a 2nd tank etc!
For those of you thinking "Its going to look awful"... yes... yes it will. However i'm renovating the room that the tank will go in, so its currently in an unused spare room!
What do you think?