Sounds like Aragonite you are changing to? When we changed over our Malawi tank, we syphoned all the water into two huge drums.. and transferred fish over into drums, ran air and heater and kept the filters running on drums... ran hose into the tank as we syphoned old gravel then emptied, took old gravel out, tank sparkling after this long process :lol: then moved the 6fter into our room, layed down the rock work and once all sussed put the freshly rinsed Aragonite in the tank, started pumping original water back into the tank and hooked the 2 big externals back up and we were away, fish as good as gold.
Good Luck and btw, Aragonite is a stunning substrate!
Mekhaela