Wow! Great comparison photo! What wattage is your MH?
Hi Jad. My MH is 400W. I set it around 24" above the seedlings. I believe the faster growth was due to several things.
The light and the added warmth it gave off.
Something for the plant to eat. I have read several posts stating that a seedling doesn't have to be fed; that you only have to start feeding when the first true leaves appear..... WELL, it looked like my seedling really liked a little food.
Moisture and plenty of airation. When I started my seedling in potting soil it compacted and the cup stayed heavy and too wet IMHO.
So the right amount of warmth, light, food and airation made the difference in MHO.
The first time I tried this 2 of the seedlings seedlings croaked... just sriveled up.. because they were set slightly too high for the hydro solution to soak the rockwhool well.