drop the water line to 25%
put the fish in a bucket with a heater and filter media etc
remove gravel add sand,
top back up
get water up to temp using a kettle, but dont touch the glass with boiling water!!!
when its at the normal temp, add filter media and lots of floss, leave it all running and wait till the water is clear
make sure water is at right temp and add fish.
make sure you wash the sand first, although i have found with toysrus sand you do not need to rinse it there is no silt in it.