Young plants's tips turning yellow

Hey guys,

First time grower here. I started growing this Sour Diesel strain from Dinafem and after about 20 days it looks like this (see pic) and it started to show signs of discoloration around the tips of the top leaves. Initially I think I over watered them a bit because it seemed like I was having a slow growth but I read up on it and and I think I managed to bring it around. It had a slight growth spurt and then the yellowing appeared. I'm feeding it whenever the top inch of the soil is dry. The lights are about 20-30 cm away from the plants.

Please help!

Setup:

- Indoor grow tent
- Lights: 2x300w CFL
- Organic Soil: BioCanna light mix in 3,6l pots (for now, will switch to 11l later on)
- Nutrients: BioVega and BioRhizotonic on normal feeding at half strength (whatever was on the nutrients schedule was divided and then mixed in the water)
- 1x120mm cilp fan on low

She's a baby! Babies need no food in good soil. Don't overwater her, but give only water the next two to three times. When you go back to feed use 1/4 strength until she has 5-6 nodes. Work up strength as she grows.

Thank you kind sir!