Is this an iron deficiency?

You should ph the water down to its lowest point for your type of growing. Lemon juice and vinegar can be used in a pinch, but you should ph the water as low as your substrate will take. Lemon juice and vinegar can be used in a pinch, but, they are not stable ph reducers. The ph will drift back up very quickly. Your problem if it is ph related will not get better. The planters will go back to their original ph very quickly. For 15 bucks a stable ph reducer is much better for your plant. Also, prepare your water 24 hours in advance to let the drifting up of the ph to be able to settle. Then when you ph down, it will remain much more stable.

fluqhing always causes problems, just dont feed anything for 2 weeks or a bit more and it will stabilise. dont panic

So, I am confused how exactly to flush because the pot is 20l , I am afraid to not over water, any advice?

it's not iron deficiency, you actually have N toxicity