Welcome to TPT!
http://www.plantedtank.net/forums/substrate/149313-changing-substrate.html
http://www.plantedtank.net/forums/substrate/150442-changing-substrate-mixing-em-together.html
threads cover many of the questions and hope they help.
Answers from the linked threads.
I use a bunch of 5g buckets.
Remove about 4g of tank water into a couple buckets.
All the plants into one, fish, driftwood, anything I want to keep wet into the rest.
I run an air stone in each bucket of fish and plants, whether it's needed for the weeds IDK.
I clear a corner down to the glass and siphon out all the water I can get.
Having a small ice scoop I use that for removing the gravel, sand, what ever being careful to stay off the sides not to scratch the tank. Dumping it into dry buckets.
Using an old credit card I push the remaining substrate to center as possible and use the scoop for all I can get.
ShopVac eats the rest LOL
I've done this by choice changing things up.
Changing the substrate keep all the tank materials wet during the swap in tank water. Put everything else back in the tank the day you do the swap to help avoid another cycle.
While doing this do not even wipe off the glass inside the tank, don't clean hoses, heaters, don't clean anything with 'tank slime' on it. Put the fake plants, EVERYTHING back in to maintain as much bio film as possible. Do not clean the filter before or for a week after the substrate change.
Add to top off the tank but don't do a major water change. Changing all the substrate is drastic enough in one day. I re use as much old tank water as I can save.
Hope this makes sense
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