Drooping leaves

Hi guys i bet this is a very common one and i bet your board of answering it, but i have ak49 its been good so far, but i always had yellowing lower leaves plants are about 7" tall and were starting to turn yellow, i read it could be nitrogen or ph problems, so i have flushed with no feed (canna a and b) and RO unit water ph neutral but one plant has started drooping like as if it has no water, ive fed it more water but not too much but still no joy, light are 600watt and about 1 and a half foot from plants i have good air flow and temps at steady 27 degres c. any help would be amazing now as im getting a bit panicy

How often do you water? What size pots?

I had the same thing with my last grow....the soil looked dry, leaves were drooping so i would water...it would look good for awhile then would droop again, so I would water more, then yellow leaves.

It became a cycle where i thought it was under-water but if fact it was over-watered....not saying this is your issue, but it sounds similar

I started to weigh my plants and wouldn't water until a certain weight.....

So my suggestion is to let it get good and dry before watering again

You could be over watering them. I wait until the soil is nearly dry and the pots feel light.

I agree with the Indoor Sun King. Definitely weigh your pots before watering. if you have extra soil and another empty 3 gallon pot lying around, fill it with the soil. If the soil is bone dry, add about 2 cups of water. Now weigh it. Next, weigh your troubled plant. Now, don't water the troubled plant again until it's down to the weight of the plantless pot.

You'll soon get a feel for the correct weight of when you should water, and you won't have to use the scale anymore. But when in doubt, pull out the scale again.

more information is really needed, container size, water amounts given and how often given. along with nutrients application rates and what you have and are giving. thanks mate hope we can help u out