My uncle gave me some corals and shells that was already dead it was kept I think 1 year from now without water does it still will affect the ph in the aquarium for a alone 1-2 inches flowerhorn?
It'll raise the pH higher than what it is. I would boil the shell and rinse is thoroughly before use with your flowerhorn. I would suggest you to put it with your filter or somewhere that won't scratch your flowerhorn.