Clean it briefly and put it in your tank. your 5 years old tank is cycled - it will not change if you put in new (old) rocks. Absolutely no need to cycle them - you do not cycle rocks you cycle tanks and your 5 years old tank is cycled for sure. I´m sorry that I am giving opposite advises than others - but why should easy thing be so dam complicated.
Sincerely Lasse
I agree to an extent.
How much “live rock” are you adding to your established tank? Since this rock WAS live...it means there’s dead bacteria and microfauna all over it. If your tank is small, or not cycled it will cause an ammonia spike from the decomposition of dead organics.
If there’s not a lot of new rock in proportion to your current rocks in the tank, I would soak the rock in a bucket of bleach for a week and then let it sit in the sun for 2 days. Letting it sit in the sun is very important because the bleach will completely evaporate in light.
This bleach method will accomplish your same goal in a fraction of the time.
Slow curing a rock like you are doing now is doing the same thing as the bleach process, but at a very slow rate. Decomposition takes time.
One benefit of the slow, natural way is that you get to build new living microfauna for nitrification.
But why would you need to waste months of your time waiting for microfauna to build when you already have a large sustainable population sitting in that 5 year old tank? Your tank can handle it and will spread its own bacteria to the new rock.
Don’t just dump the rock without bleach as above user said. You will cause nitrates and phosphates to leach and cause a bloom depending on how much organics were left behind in the old rock. But bleaching it will remove the dead stuff.
