The pump of my calcium reactor feeds from the buttom and the outlet is in the top of the reactor.
Does this will work effectively or it must be the other way?!
I can see that it is a down flow reactor. If you use a fine media it can compact and become less efficient. Just use large media and you should be fine. When you need to add more media make sure to rinse any sediment and small particals out of the old stuff before putting it back together.