Dangers of ancient substrate?

[mindfestival]

Hi Gents,

Substrate is about 2 years old, aquarium gravel.

Enormous amounts of mulm from top to bottom, and virtually undisturbed apart from the odd vacuum around the plants.

Apart from aesthetics, does there come a point where there is an excess of organic matter in the substrate, thus offsetting any nutritional benefit for the plant?

I have no nitrate excess problem.