Re-posting as a new thread to get better response

I sowed Petunia's for the first time ( ok it was my first seed sowing too :) ).

All went well,
seeds sowed on 24-Aug
germinated - 27-Aug
transplaned into cups - 06-Sep (immediately after the seedlings had true leaves)

Today 19-Sep they are not big enough. They just have an extra set of leaves.
The seeds were sowed in coco peat cups and are in open. Should I fertlize them.
I have a water soluble fertilizer which says NPK- 18-18-18. The instruction I got was dissolve 5ml in 1ltr. Is this OK?

The temperature in my city is : 90�F
Precipitation: 70%
Humidity: 65 %

Need guidance on fertilizing. Thanks in advance.