Help With Deficiency Please

[badseed]

I currently have 4 plants in coco all fed from the same reservoir. I switched them to 12/12 almost 3 weeks ago and one of them has gained over a foot in height above the others and seemed to be going great until today. I have not changed the feed for a week now, but in the last 24 hrs a lot of the lower leaves on the larger plant have turned yellow and some of the top leaves are showing brown spots around the edges.
None of the other smaller plants are showing any of these symptoms at all. The ph in the reservoir is 5.8 before feeding and generally after feeding when the run off returns the ph is around 6.1
The temp is a pretty constant 80 degrees with lights on and 66 overnight. The lighting is a 900w Mars and a 600W hps. The plant that has stretched the most is predominantly under the led.
Any input would be greatly appreciated, thanks

 

[badseed]

The second last pic is a height comparison against the other plants and the last pic is just the other plants that seem healthy

 

[Aqua Man]

I'm only taking a stab at this and don't take my advice without someone confirming it. With coco and this stage of growth and the pics I personally would say 99% Calcium deficiency. Coco is notorious for its effect on calcium uptake. I would suggest some calmag. Again please confirm this with others.

 

[badseed]

I'm only taking a stab at this and don't take my advice without someone confirming it. With coco and this stage of growth and the pics I personally would say 99% Calcium deficiency. Coco is notorious for its effect on calcium uptake. I would suggest some calmag. Again please confirm this with others.

Thanks, appreciate the reply. Have been feeding with calmag in the reservoir. Maybe I need to change the feed over a bit sooner. I notice the EC in the tank drops dramatically over 10 days

 

[Aqua Man]

Thanks, appreciate the reply. Have been feeding with calmag in the reservoir. Maybe I need to change the feed over a bit sooner. I notice the EC in the tank drops dramatically over 10 days

I'm not sure what type of coco you have or whether its buffered and how well. Here is a read that explains it pretty simply.

Maybe a flush with only calmag? this is something someone with more experience would be better to help you.

 

[Aqua Man]

Kinda looking like an N issue too so its possible its just overall nutes being low. Old leaves will not repair themselves so even once its fixed it will not change those leaves.

Anyone able to chime in on this who could be a lot more reliable of a source for him?

 

[badseed]

Kinda looking like an N issue too so its possible its just overall nutes being low. Old leaves will not repair themselves so even once its fixed it will not change those leaves.

Anyone able to chime in on this who could be a lot more reliable of a source for him?

Thanks for the input mate, really appreciate it. I've cleaned out the reservoir and upped the nute concentration a little, so I guess I'll just see if it gets better or worse, cheers

 

[PharmHand]

Try mixing up only enough nutes for one watering at a time (certain minerals precipitate out of solution)and bring the night temps up to 70+