Adding sand to established reef tank

[sunoka]

I have 2 reef tanks on the go for the last 4-5 years. From vacing the sand and cleaning over the years the levels have dropped and need to be topped up.
I have sand from a couple of tanks that I shut down 2 years ago, now what is the best whey to do this can I slowly add small amounts over a few months or should I try to cycle the sand in a couple of tanks. I have 5 gal pale full of sand I need to cure. Input please I don`t want to screw this up my fish and coral are to important to me to take risks.