Are the beds fed in parallel or in series? What's growing in the different beds? Is the flow rate the same? Any testing to see what the nutrient levels in the water are? My guess is it has more to do with the type of plants you're growing but we may find out otherwise.

Cheers

If you're using commercial fish feed then you probably don't need to be concerned with phosphorus because it's in the feed so it's just getting used or bound up as fast as it's going into the system. Count your blessings that the phosphates are zero if they weren't you'd probably be having an algae bloom. Phosphate is very low or zero most of the time in my AP system.

I'm not sure I understand what you mean by the beds being in series. If they are in series, what comes out of one goes into the next so then the flow should be close to the same through each bed but you state that it's not ?

Usually the first bed gets more nutrients if the beds are in series and this results in the first bed growing a bit better but sounds like your setup doesn't give you this. Check the water temperature - some of your plants are cool weather plants and some like it warmer (Basil). Any difference in the light levels between the beds?

OK, so based on that description of the flow I'm going to guess that bed 2 and 3 have more flow than bed 1 and that's why they are doing better. This is speculation on my part but it may be that there is one nutrient that is slowing bed 1 up but because more of the nutrients pass through beds 2 and 3, they are getting enough. FYI - Basil is a warm weather plant so it might be slow because of cool water temps. Post up some pictures of the plants showing which bed is which and I might be able to tell what's going on.

You can use ball valves at each grow bed to regulate the flow and even it out if necessary.