Yes buddy epsom salts is the one. I've just had a quick look at the TNC stuff and there is the same amount of magnesium in both lite and complete albeit in small amounts. My understanding of using Duck weed to look for deficiency is new leaves would be iron, old leaves potassium and nitrate which the plants consume the most of. That just leaves magnesium which affects old leaves also. Potassium and Nitrate can also cause stunted growth in new and old leaves but yours look reasonably healthy. I would remove any poor foliage from under the leaves and have a shot with the magnesium, I think magnesium is mobile so you should see the leaves green back up if that's what you're lacking.
@dw1305 is really the man for this one though, Duck weed index is his baby.
There is a law whose name escapes me right now, liebrig or something or other? It dictates that plants need all the trace elements plus N,P and K to form healthy plants if any of the above is missing it weakens the plant, look at it like building a wall without sand. That's why it is better to use a complete style fertiliser and ignore your water report. If there is some nitrate of phosphate coming out your tap then happy days you're belt and braces. Or you could save yourself a few quid and just buy the raw salts and mix it up yourself. Being a chemistry teacher you'll probably know better than most
Regarding EI, Basically all those elements we mentioned earlier are put into the tank in amounts that the plants couldn't realistically use even under maximum growth and lighting. It is really for the high tech people who don't want to run out of ferts while they are pushing growth to its limits so they can concentrate on getting flow and co2 injection right. Us low tech users just need to make sure there's some of everything. That's where the Duck weed comes in handy, it's close to the light and doesn't get co2 issues as it gets it co2 from the atmosphere and not the water column which we all know is in abundance in the atmosphere
Duck weed reacts quicker to fert issues than the submerged plants so it's essentially a canary for the rest of the tank. 👍
Just to add, if it is nitogen you're lacking and there is a lot of nitrate in your tap water doing more or bigger water changes would help.