Need a little help or confirmation

Hi

I need some help, this is my third grow with the same set up that i'm using and had no problems until now.
I'm growing in coco and under a 600w HPS, airflow is good.

My problem is or at least i think it's over fertilization (nute burn), i have black spots on the leaves which have developed over the last week but beforehand i noticed quite a bit of yellowing of the leaves.
I have flushed my plants twice but haven't really seen any improvement, they are still growing slowly, about 1" in a week and the new growth has a pale yellow colour to them on the tips.

My first two grows were from clones with no probs and these are from seed (white-widow) and about 4-5weeks old, started to fert at week three 1/4 strength. They stand at around 8" on average. I'm sure i've over fertilized because i upped their dosage by two fold because i thought they were big enough to handle it.

Here's some pics;


Any thoughts or suggestions?

THX.

Sorry for posting in the wrong section, feel free to move admin.

Hi
I have been growing for about 3 years now, Had a problem with nute's yellowing leaves etc, since switching to bio canna organic nute's have not had any prob's (you cant over nute with organic nute's) also Top Max is good stuff ! I also like to use a good quality soil Canna or Bio bizz have been good for me, As with soil you have a natural filter so you have less chance of nute lock also, Always keep to what it says on the label too. Hope I have given you a lil help
They look really good so far Good Luck

Yeah, thanks man.
But these plants are doing my head in mate, any ideas what could be causing them to react the way they are?
Yellowing and getting black spots?
As i said i may have over ferted but i have put that right now and im getting good PH on the run off, do you think i may have over ferted them and now after about two weeks im under ferting them?..hence all the yellowing and signs of deficiency?