This Old Spouse
TOTM Winner May 2013
If you wash the sand properly it will only cloud the tank very slightly for a few hours, at most.
If the tank is reasonably new and there hasn't been time for too much mulm to accumulate, you might be able to change with the fish in. Syphoning the gravel, and any associated crud, out with a wide bore pipe, like a piece of hose would be your best bet.
From an opinion point of view, do you think Sand would look better in the tank? From the picture that is.
Definitely would look better, and much more natural.