Hi - oh that's good you managed to get some sand! You'll love it I'm sure!
From my recent experience I would say if the tank you're working on has been established for quite some time and is over 2ft, it's best to remove the fish if you are taking a gravel substrate out.
The reason being...hidden gunk! I'm glad my fish weren't in the tank at the time I took my gravel out (my situation was a bit different in that I was upgrading and had already moved my fish over to their new home). Regardless of the weekly gravel vacs I'd been doing, I found to my disgust that as I removed the gravel it was full of brown gunk right underneath, especially in areas that the gravel vac couldn't reach easily. Urgh! There was about 10% water still left in my tank at the time and it went a horrid dark colour. Imagine if the fish were in there!
Obviously, it's up to you how you want to do it but that's just my experience.
If you do go down this route, I would put the fish in a large (clean/new) bucket of tank water, with the heater and filter in with them. They will be fine like that whilst you quickly work on the tank.
You could remove 50% of the water, then scoop out the gravel (I used a large strong net). You might be lucky and have cleaner gravel than mine (mine was 3yrs old) that doesn't mess up your remaining water. In my case it didn't really matter as I was closing down that tank. But if there is any sludge you might just want to empty the tank, clean it out a bit before adding your sand and re-filling.
Depending on the size of your tank it can be quite time consuming but so long as you keep your filter running in the bucket (to keep your media alive) and the heater on to keep the water at a stable level, the fish should be fine. They may be a little peeved off and sulky for being uprooted but they should be fine once you put them back in the tank and they've had time to readjust.
Obviously, doing all of this can alter your tank stats - even though you have your entire cycled filter in-tact, the equilibrium would have been disturbed within the aquarium and may take several weeks to settle back down again, so water checks and changes to keep any sudden spikes down.
Well, enough waffling from me LOL - good luck with it all and let us know how you get on!
Regards, Athena
