Problems and solutions?

Although growth of the plants in the growbeds is quite vigorous they don't seem to be doing as well as I had expected.
I am reasonably convinced that I have 2 main problems:-
1. not enough light and
2. not enough nutrients.

So in an effort to solve or at lease improve on the light problem I decided to install the extra polycarbonate roofing I had bought at the beginning of the construction.
Taking advantage of a visit from my "nephew" I co-opted Gary into being a roofer's mate and we removed the zincalume roofing over the back part of the shed over the fishtank. This is a reasonably straightforward job despite the difficulty of trying to drive "one-shot" tek screws into polycarbonate sheeting whilst not putting any weight on the sheeting. Fortunately the sheeting is very strong and the occasional accidental step on it doesn't result in a crash through to the ground 4 or 5 meters below.

The fish don't seem to realise that the extra light is good for the whole system and they are currently much more skittish than they were before the extra light became available. I am hoping that they are just getting used to it and will eventually be more willing to be seen.

The not enough nutrients problem is almost certainly due to the fact that I have quite a lot of plants and not enough fish poo! There are probably over 200 fish but the biggest of them are still only about 100mm long- and a lot are much smaller.
So as mentioned in an earlier post I am adding pure Urea at the rate of 4g most days- this has lifted the nitrates to between 10 and 20ppm without causing any detectable ammonia or nitrites.
I was also adding chelated iron at the rate of about 5g per day for a couple of weeks. I was testing for iron in the water but not getting any detectable levels. However I should have been testing for the chelated iron and when I finally did so the levels were between 1 and 2 ppm. This is possibly too high a level so I am no longer adding iron. A test today suggests that the levels are a bit lower and I hope that the plants will take up more. As far as I can tell chelated iron is not toxic to fish and the fish don't seem in any way affected.

The fish are also eating less now due to cooler temperatures.
Temperatures in the shed are now a fair bit lower than they were in the middle of summer- not a surprise- as I write this the temperature outside is only 10 degrees Celcius. The tank was at about 15 degrees earlier today. The insulation I have around the tank is clearly helping to moderate temperature swings.