why is my phytoplankton not dark like others

[The Guy]

Originally Posted by donkey77

temp is about 72 74
airstone to a slow boil
this is my first batch
3ml f2 for about 3 gallons

I was told not to use an air stone only a hard tube open ended at the bottom of the bottle, also I use a 6500 k compact bulb.
I'm new to this so we'll see how it goes.