Water, and food

In the first week, you can start moistening the areas around the seedlings with 300ml of water every other day, as they don't absorb much at this stage. Then, after the first week, you can increase your watering dose to 600ml. In week 3: 900ml and week 4: 1.2ltr. The substrate, as mentioned, can hold up for 4 weeks without applying nutrients. Adding potassium during flowering is essential.

Miracle soil will basically give you like around a month of nutrition for your plant, no need to supplement. With nutrition it depends which route you want to take - bio fertilizers or mineral ones. I'd recommend getting a basic starter pack for growth and bloom - BioBizz Bio-Grow/Bio-Bloom/TopMax is what I personally use, you can also get similar products from other companies like AdvancedNutrients. When it comes to watering in the seedling stage - give it just enough water to moisture the area around the plant. I'd also recommend keeping the rH levels at ~60% at this stage, you can also make a humidity dome from a plastic bottle and that will also do the job 😇