which deficiency?

First of all nice air pump. I have the same one and it's a gooder :). Get the Liquid drops.. They are far superior to meters.. Just because the drops are cheap, never out of adjustment and actually require you to physically look at your water. in your water. Root rot happens later usually unless you have bad stuff in your water to begin with. Check your local water report.. If I knew what you were feeding I could probably advise.. My best guess is your feeding Cal-Mag which can cause this.. It's an over dose on calcium or it's from your ph being a bit low too often.. Higher is better then too low. ie 5.8-6.5 is a check guideline but try not to let that ph dip below 5.8.. The more the Ph moves a bit the better.. If your ph is moving your plant is transpiring and using the water. Plants put Acids and Bases into the grow medium to help intake the nutrients they want.. So if you run in and drop your ph to 5.8 every time it goes to 5.9 you may in fact be "fighting" against your plants... I myself was "amazed" at seeing other locals gardens growing in soil.. They had use no nutrients or ph buffers just water and good soil. And there foliage looked pristine and amazing. I wondered why they with so little effort could have such great results when I saw so many online struggling with soil and growing in general. I learned that a lot of the information you get online is Informative. but It overcomplicates things and takes away from the fact that plants are living beings.. They want to grow.

I think it's boron deficiency... You may need to check the pH of ur soil and raise it up... Try watering at 6.2-6.3 instead... Better nutrient uptake