Changing substrate

I did mine with fish and water still in, I highly suggest you gravel VAC the hell out of the substrate before removing, or it'll kick up a s*it storm of dirt!

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What do you plan to change the substrate to?

If you don't want to break down the tank, then just take out a small portion of the old substrate each week during water changes or whenever you do water changes. Just remember to turn off all the filtrations and leave it off for about a half an hour after to let all the sand settle. It'll be a slow process, but you wouldn't have to breakdown the tank

I would recommend not removing all the substrate at once since it does harbor beneficial bacteria, and depending on your set-up, it could be significant

If you just bag up some of the old substrate and put it back in with the new stuff, you won't have to worry about losing too much of your bb...

My suggestion would be to: pull your stock and large deco out and put them in a crate or two with tank water. Thoroughly gravel vac your existing substrate and pull it out (I prefer a dust pan to pull for this) and bag some of it up (ladies nylon stockings will work in a pinch). Then just add the new substrate you'd like to change to, do a large water change and add everything back to the tank. It may take a little time depending on the tank size but as long as you keep the deco and bagged substrate soaked and have a good amount of bb built up in the tank and filter(s) you shouldn't experience any sort of mini cycle. I've done this on a few tanks, and have never had a problem, just make sure to leave the bagged (old) substrate in for a week or so before you pull it out.