Despite some theoretical arguments to the contrary, in reality Flourite never needs to be replaced. Unlike some other substrates that are "charged" with nutrients before packaging, the iron, calcium, and magnesium in Flourite are integral to its structure. They are considered entirely insoluble, chemically speaking, and can only be acquired by plants through enzymatic action in the root zone. They absolutely do not leach into the water column to be removed through water changes, and there is enough in the structure of the substrate that it would be centuries before plants could utilize it all.
The porosity of the substrate can be considered the same way. It is integral to the physical structure of the compound. It doesn't collapse or break down, anymore than cement (which is also porous) will dissolve away when submerged. It simply doesn't happen in the real world.
Since the nutrients in Flourite are only available to the plant roots and do not leach into the water, supplementation with liquid products is still recommended. Flourite feeds the roots, while the liquids are available to the stems and leaves of the plants. Taking that one step further, the liquid products are generally not available to the roots of the plants, so if you want the root zone to have a full complement of nutrients outside the mineral composition of Flourite, you will need to use some root feeding products, such as Flourish Tabs.
Hope this helps! | 
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