Update 2012/12/23
In this post I examined and searched for the cause of yellow mottling on these spinach leaves. Finally I found that the cause was an Iron deficiency. In the months that followed I posted several more articles with good information about this and other nutrient related signs, problems, and solutions.
For a scientific explanation of the chemistry behind the bio availability of Iron go to
[Are These Rocks Nutrient Theives?]
and
[Assimilation of Nutrients]
For a detailed yet condensed collection of charts and facts go to Nutrient Deficiencies
Below is a record of my wondering search while I learned what I needed to know. I would prefer to simply delete it, but you may relateto my learning curve and find something interesting along the way.
Update 2012/06/07
Im still having problems with yellow leaves. I have brought my PH down from 8.2 to 6.6 and I have no salt in the system I have also added 1/2 a bottle of Microbe Lift Chelated Iron. Ill Send an update in a few days with the results. I have more iron on order as well as a GH/KH test kit. Hopefully I will get this figured out.
Update 2012/04/14
"A plant injured by excessive sodium first displays mottled leaves or yellowed tissue between the veins of leaves. This is followed by leaves that are dead at their tips, at their margins, and in areas between their veins." - WateReuse Foundation
My search for the cause continues. I had not seen any insects so I hung sticky cards to trap them. All I got was two mosquitoes. Several weeks ago I pushed my salt levels up to 0.35% because I had a few fish that were damaged during the transfer to the aquaponic tank.
Ive been bringing the salt level down slowly and it is now at 0.20%. In the past when I bumped salt levels up in my pond I did not notice any damage to my ornamental plants, but Spinach and Tomatoes may have a lower tolerance.
I will begin to lower the salt level over the course of the next week and see if the new sprouts show any of the same signs.
Further information:
http://salinitymanagement.org/Salinity%20Management%20Guide/index.html
Update 2012/04/12
After receiving assistance from Zalinda Farms Inc. , I now believe the problem may be caused by Thrip.
If not this information is still a value. Keeping a vigilant eye on the crop is always important and Thrip is just one of condition to look for.
I have yet to actually see the Thrip, so I will purchase some Sticky Cards. If I find Thrip I will use Monterey Garden Spray to control the pests. The literature indicates that it is a safe bacteria.
Natures Control also offers several solutions. One is Predator Nematodes
but in an aquaponic system this may not be an option. The instructions state "The important things are to use them within 2 hours of mixing, because after that they start to drown". ... " Theyll live longest when the soil stays moist, but not saturated with water."
I suspect they might be washed away into the water each time the grow bed ebbed and flowed.
"Thrips Predator Mites (Amblyseius cucumeris) are most effective under conditions of 70-85% humidity, against all species of Thrips. They will also eat an occasional Spider Mite, and other small pests. However, reports have been poor in low humidity environments, so use these predators in greenhouse and other interior locations with high humidity levels only. "
Two types of sticky Traps are also offered by Natures Control. They say "Customers report success using Sticky Blue Traps against Thrips & Leafminers. Especially recommended for use on roses."
I found further information at
http://biocontrol.ucr.edu/wft.html
The definitions of INSV and TSWV are as follows:
Vectoring of plant diseases. Western flower thrips is a vector of many plant diseases, the most important of which for greenhouse producers are two plant viruses in the genus Tospovirus: impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV)
Original Post
I wonder if anyone can help me diagnose this problem Im having with my spinach.
I suspect either a lack of light, salt burn from treating my fish, or a bug, but I see no bugs.
My spinach leaves are mottled with yellow. 
The orange arrow points to a very small white area that I found behind the stem of a leaf I removed. It looks like foam.
The underside of this leaf has very small round white spots. This picture is through a magnify glass.
Home Aquaponics: 1 dead fish and flowering tomatoes- YouTube
Ok Folk, Hope this helps or encourages or pricks your interest. And if You have the time take a look at the Video below......