Yep!... Arbe's eyes aren't failing him, definitely aphids. Can be easily treated by spraying with Neem oil. Just ensure you cover any exposed areas of gravel in your grow beds with newspaper 3-4 sheets thick prior to spraying. Also ensure you cover your fish tank and/or sump tank so that spray drift can't get into the water.
Why are you dosing your system with Iron every two weeks?... You can add too much Iron, it can cause issues with other elements. I have a single IBC system with a 500L water capacity and I only add a "pinch" (quite literally just a pinch) of EDDHA chelated Iron every 6-8 weeks, and my plants are extremely healthy, see the pic below, taken this morning. The system has been running 5 years and for the first 3 years or so I only added Iron twice, when I actually had Iron deficiency symptoms showing, and I still had excellent plant growth.
From the pics you've provided it's difficult to diagnose which element your plants are deficient in. Pictures of the overall plant/s would help, it's important to know where on the plants the symptoms are showing, this helps diagnose which element it may be.
But as a start, I would stop adding Iron so frequently and start adding a capful of seaweed extract (ie: Seasol or Maxicrop) per 500L, per week.
Also, what is your system water pH and temp?... and what form of Iron have you been adding?
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