First for treatment of the new sand, keep it in a large tub and wash it with
water while stirring it with a stick till the water above clears
and u can seen the sand through it.
Then for acid wash, buy the HCL(muratic acid) used for bathroom cleaning
and pour some amount of it in the same tub.
The sand will fizzle. keep on pouring some amount & stirring till the
sand stops fizzling. Then again wash the sand with plain
water till the acid smell goes away.
While using acid, take due precautions to avoid acid burns,
e.g., use gloves, keep children away, don't shake vigorously and splash,etc.
For adding the new sand to the tank, get a large 1or 2 -inch diameter
funnel,
attach a long (e.g..3 feet) plastic pipe to it and then position this into
the tank where you wan to introduce new gravel.
Then slowly pour new sand though
the funnel and slowly remove the pipe upwards,
so that new sand does not cause a storm!.
You can either remove some old gravel or just pour this new sand
where you think there are gaps in the old one.
I don't think there is any harm if you bury the old detritus under this new
gravel, and further it will act as fertilizer.
Hope it helps
regards,
ajit athale
India.