Do a water test for nitrites and nitrates. A pH of 7 is perfect for AP. Fish prefer 8, plants 6, so a pH of 7 keeps both happy. Yellowing of new growth only usually indicates an iron deficiency. Yellowing of old growth or all growth a nitrogen def. How are the fish? Plenty of light? Try adding maxicrop with iron to you water, it will cover the micro nutes, and iron def is common in AP, because fishfood doesn't have any. Don't expect great results from aquaponics until system is matured, and stable for 6 months or so. Each 5 gallons of water can have 1 lb of well-fed fish, and 10 gallons of media that stays both wet and oxygenated. If you're outside of this ratio, then things won't be optimum. Not enough fish or feed, then not enough nutes. Not enough media, then not enough bacteria to convert fish waste and things go south quick.