When is the best time to start dosing your aquarium with fertiliser?

When is the best time to start dosing fertiliser?
Have you put any plants in your aquarium? Then start dosing!
Even if you have a nutrient-rich substrate, such as aquasoil or a DIY dirted solution, you need to dose liquid fertiliser to your water column. 
Why is that? These enriched substrates do hold a lot of nutrients and will be leeching ammonia and/or nitrates for the first few weeks after setting up your aquarium, but won’t be doing so at a rate sufficient for your plants’ needs. These substrates typically don’t supply enough potassium or micronutrients in the early stages either.
Put simply, without water column fertilisation in these early stages, your plants won’t be receiving all of the nutrients they need to grow without deficiency.
Another benefit of fertilising the water column in these early stages is that your plants have not yet established root systems, so they won’t be able to uptake nutrients very effectively through the root feeding zone. The great thing about aquatic plants is that they can absorb nutrients through their leaves too! 
By providing them in the water column via fertilisation, plants will have nutrients available to use immediately, allowing for faster root development and growth. These nutrients will be transported quickly from the leaves to growth tips, much faster than via the roots.
This also eases stress on the plants as they adapt to the new environment of your aquarium, letting them transition smoothly and start growing with minimal melting.