A distinction should be made between 2 functions of the substrate:

The direct fertilising function of the clay and the added trace elements should be between 1 and 3 years depending on the granularity of the substrate and the degree of through-flow (floor heating).

After this time, the effect can be occasionally replenished with Terrdrakon NPK/SE balls aquamax.

The improvement in a substrate generally consisting just of quartz gravel or sand with clay to a substrate which is much better for plants is retained even after this time.
Clay minerals (layered alumosilicates) have a similar function to ion exchangers and buffer various fertiliser substances, i.e. in the case of a surplus they absorb them and release them again as required and thus lead to a long-term improvement in the function of the soil.

A regular replacement of the substrate due to depletion of the Terrdrakon substrate is therefore unnecessary in our opinion and need only be carried out if there are other reasons for re-fitting the pool.