Yellow leaves - Nitrogen??

This is a N-deficiency. As certain as you can be about any visible symptom. This one is unique enough as opposed to others that overlap quite a bit with visible symptoms. Can eliminate other possibilities mentioned. If it was pH, it'd fuck with Ca long before N and potassium or phosphorous deficiency doesn't look like this. https://www.reddit.com/media A high-resolution leaf symptom chart you should save to your computer or phone. Can zoom in for a clear picture. As you can see potassium would be a problem around the leafs serated tips first before becoming worse... interveinal chlorosis is also different than how chlorosis related to N progresses. P deficiency would have blueish leaves and dark copper spots. i see you are in flower. Most 'marijuana' fertilizer brands often drop N way too low in their feeding charts for bloom phase. It wouldn't be unusual. Whhat else is in fish mix? if it doesn't impact other stuff too much. definitely amp it up. How quickly this progressed should dictate how much you need to adjust... slower the progression, the smaller adjustment needed.

Starving for potassium (basically, under fed)...........also hungry for phosphorous and no doubt, a small amount of nitrogen too. Why join Grow Diaries and then not have a diary, especially if you want good adivce?

Looks like nitrogen deficiency, could also be an issue with ph levels and why they are not uptaking N. Maybe check the runoff water, and if it's ok give them a bit extra nitrogen.