No substrate?

[D.Wolf]

If you remove fishes it will be less stressful for them..but if you do the sand change all at once, it could have impact on filtration (as sand is kinda part of bio filter too)..if there is option, divide your tank bottom on 3 parts and every few days change one part of the sand..

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[BassetsForBrown]

if i were you,i would get the sand first.then start the process early in the day.put fish in buckets while you work with an airpump running to the fish buckets.i think it's less stressful to the fish to complete it in 1 day

I wouldn't be scooping it out of the tank or anything. I have a 3/4inch wide tube id use as a siphon, taking the water and the sand right down the drain together.