Deficiency - P or Ca maybe?

[BAMWW]

Hi all. Any idea what's causing these spots and yellowing of fan leaves?
I thought it was P deficiency as it's well into flower and I've only used Biotabs so far but the Green Sensation doesn't seem to have cured it.
I don't want to try anything else if it will only make it worse!

Problem: Rusty colored spots on fan leaves which then turn yellow.

Medium/grow method: Biobizz Lightmix. PH in/out both 6.5 at last watering.

Feed: and supplements used: 2 biotabs in a 12L airpot. I've started adding Plagron Green Sensation (PK13/14) the last couple of waterings - going from 0.25ml/L to 0.5ml/L at the last watering.

water source: Rainwater/RO water

Strain/age: FastBuds Orange Sherbet/ Day 54

light used: One 50w Cree COB per plant

Climate: Temp approx 26C/Hum 55-60%

Additional info: This plant had a nitrogen toxicity issue several weeks ago followed by some of the rusty colored spots but this cleared up.

 

[Proph]

So two bio tabs and that's it? What about the other products that work along with the tabs?

 

[Mañ'O'Green]

PH for Bio-Bizz is 6.3. Your rain/RO water is too soft so you need to add a little calcium. 50 PPM?

 

[BAMWW]

I'm on a limited budget at the moment and couldn't get the whole biotabs kit. I already had some Plagron nutes and the guy in my local shop said the tabs should work OK if use the green sensation during flowering.
To be fair it has worked out OK so far ameven though I had some issues that were entirely my own fault earlier in the veg phase.

I will check my water PPM again tonight but IIRC it was even less than 50, maybe only 30PPM so that could be an issue. They haven't been given any cal-mag except for one or two waterings a couple of weeks ago. Maybe next feed try 1/2 strength cal-mag and also water at lower PH to see if that helps?