I would like to know if I need to add any fertiliz...

Must Provide Nutrients from the Beginning – Coco coir is an inert growing medium with no inherent nutritional value, so growers must provide nutrients and manage pH throughout the grow. Cannabis plants grown in coco coir tends to need more calcium compared to other growing mediums, so usually a standard “Cal-Mag” supplement is needed to make sure plants get all the calcium they need. These contain calcium as well as magnesium (which is needed for your plant to be able to use the calcium properly). So I would start at 1/8 strength and work to 1/3 max of manufacturer recommendation.

Canna Terra is mostly a peat based organic soil mix peat is naturally acidic they add lime to correct the ph it also contains a starter charge of nutrients good for about one weeks of the seedlings life. Canna recommends using their Terra line of nutrients along with their feeding chart. Since not everyone counts their weeks the same way and you are starting/ending week 3? you should be able to start with 1/4 strength nutrients to play it safe. If they respond well after a few feedings by showing some nice healthy growth I would slowly increase the nutrients. The biggest issue with seedlings and young plants is keeping the soil too wet so allow them to dry a bit before they are watered/fed again. If re-potting them in the future i would recommend adding some extra perlite to the mix to allow for better drainage. If you would like to amend the soil with earthworm castings to add some beneficial microbes to your soil etc that's fine but not needed if your going to be using additional liquid fertilizers to feed them. Some grower will use a so called super soil to keep things on the organic side if that's what your looking for.Hope this helps👍

There is nothing but lime to make a good ph fluctuation issue occurs 👍 so I really discourage you to go that way except if you want to have a lot of problems because lime is temporary flowing the ph but the effect is lasting very fast : buy a liquid or a dry ph down product. Your soil is a good soil but not a super soil so you'll need to feed even if it's only the half of the recommended dosages BUT during at least 2 weeks or 3 your plants will be fine with only water as the medium will bring fresh nutrients ( later you will have to bring more by feeding )