What's causing this ?

Does anyone have any idea what's causing the tips of my plant to turn brown ? I've ruled out heat stress because these leafs are the furthest away from the lights and are the closest to the fans. I'm thinking it could be calcium or potassium ? The plant is about 4 weeks into flowering. I have the bio bizz nute range and have only recently been giving it the 'Grow' nutes because i had a nitrogen problem. Is it time to start giving it 'Bloom' nutes ?
Thanks in advance

its always a good idea to give them balanced nutes from the start...just more N and more P K in flower. still looks like your burning them

gobskiii said:

its always a good idea to give them balanced nutes from the start...just more N and more P K in flower. still looks like your burning them

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For this plant i've been real easy on the nutes as i've over nuted before so didn't wanna make the same mistake. I only gave it grow nutes when it should signs that it needed them, over than that just been giving it plain water.
What do you think i should do ? give it bloom nutes or just keep giving it plain water ? I've gotta water it later - i let the soil get nice and dry between watering

If the tips of the leaves are the only part burning, then you're your using your fertilizers just right imo.

so...how much have you fed them? and what kind of soil are you using? that could have something to do with it...if it is like time released MG or even just plain MG that would be an issue.

well it's MG potting mix - says on packet treated with MG plant food but no time release stuff in it. And i think i've only feed it nutes 5 times in the last two months and then it's only 1ml - 1.5ml solution in a litre of water (bottle says 4-5ml per litre)
I've only had signs of nitrogen deficiency so far, so only feed it food till that had cleared up.
Should i just keep giving it water ? if i need too i have 'vitalink flush'