Best way to switch aquarium substrate?

[Runsoncoffee]

My 75 gallon is a mix of several tanks through out the years. I’m looking to give it a good make over so it looks like it’s all one idea. I’m wanting to switch my current substrate (a mix of everything under the sun, sand, gravel, river rocks) to a mix of black and onyx sand. What would be the best way to do this without hurting the fish or killing off all the good bacteria?

 

[LaloJ]

Without a doubt, you will have to empty the tank and place the combination of substrates that you want to use, you can take the opportunity to rebuild the aquascape, it is fun but laborious.

 

[Aiken Drum]

Ive changed substrate before without emptying and leaving fish in the tank. You can easily syphon out sand without disturbing stuff too much. You might be able to syphon out gravel too, but scoop it out with a net if it wont syphon.

You can replace sand without removing water also using a bottle.

https://youtu.be/VfkkunBi3V0

Replace it in stages. Say 25% one week, 25% the following week etc etc.