As long as you are not using huge amounts of Maxicrop, I'm not too worried about salt build up from it. I've salted my system up to like 3 ppt every so often when dealing with fish disease or excessive fish handling and that salt level will go down slowly. Plants will use small amounts of sodium. Now if you were adding lots of maxicrop every week it might mean a build up of some things but I've added like a quart of maxicrop per month into my big system and even with occasional salt added that system will still reach very low EC readings over time (before I salted the last time I was down between 300-500 ppm of salt) Granted, I'm adding a quart of maxicrop to a system that has 900-1000 gallons of water resident at least and 1400 gallons of gravel full of plants.
And Izzy's question there is rather important to helping figure out what the deficiency is.
TCLynx, plants can replace up to 90% of their potassium uptake in the form of sodium!!! In fact, plants that do this are typically more succulent, in other words more resistant to drought. It does depend on the plant and specific genetics in question, but studies have definitively shown this. This is old news to most botanists and biologists, but I'm only recently learning this type of advanced science; so, I am still pretty excited over neat facts that have higher implications in aquaponics than in traditional horticulture.
A note on the salt: some plants a very sensitive to salt whereas some are very tolerant. I would have to check the numbers again to be sure, but I believe the plants that have some tolerance can handle 3 ppt whereas the sensitive ones may not or have stunted growth, etc. Obviously, treating for a disease in the short term may not have noticeable affects on a plant in the long term... so, as usual, it all depends on your situation as well.
I am still leaning towards nutrient deficiency as the cause of this, but I would like more information. We have not ruled out pest or disease either...
salt will evaporate when the water evaporates yes, but i'm pretty sure I read that it builds up in your growbeds.. hopefully someone on here with a little more knowledge can vouch for this..
as far as your question Izzy.. the transparent brownish spots usually form at the back of the leaf, vertically along the spine and at the tips of the leaves... my indoor garden is all types of lettuce
Actually the salt doesn't evaporate with the water and this is why it's possible to distill fresh water from salt water. This is also why if you are using lots of salt in a system (like hydroponics where the nutrient solutions are all types of mineral salts) as water evaporates it tends to leave behind higher and higher salt concentrations in the grow media.
However having said that. We are doing aquaponics, not hydroponics and the amounts of salt we use are normally far lower and we are not adding them all the time. (or at least most of us avoid adding lots of high salt content things to our systems) So the plants will use up the salt from say salting your system to between 1 and 3 ppt of salt when introducing new fish especially if you don't salt again till it's time to add new fish again next year.
If you are constantly adding salt in the form of huge amounts of maxicrop (say a weekly dose of an oz per gallon) or constantly adding sea water or sea 90 or using baking soda to adjust pH well then you probably will see salt build up to a problematic level.
I probably add an oz of maxicrop per grow bed per month to my system.