yellow fan leaves

hi guys ive 3 in my friends loft we have 3 mango cross ak47 one tall 1 4 foot has started to get yellow tips on bottom fan leaves it has slowly spred up to the next fan leafs the other plants are smaller and stocky they have onlt few yellow leves they are into flower 4 and a half weeks in 10 litre pots i just got 15 but dont want to repot coz of stress is this just the leafs starting to die off coz growth going into the bud now
any advice

have you been feeding them?

yes they av been just fed there first guano tea lastnite i was hoping this would help them b4 that they were on bio bizz

well its natural for the leaves to turn yellow during flowering stage. whats your soil mix?

gold label from amsterdam pre mixed supposedly the best so people say