What Konsa said mate, switch to something with N and P, a tank that size dosing dry salt will be far more cost effective.
The problem I have is plants start to get some black algae on leafs
Plants aren't thriving so algae are gaining a foot hold, the yellowing leaves are a sign that you may be low on nitrate so not thriving=algae. You maybe also need to up the Magnesium via epsom salt.
I used to dose Iron and Potassium but had some fish die perhaps not tolerant on these .
Will be totally unrelated mate, just one of those coincidences. Plants need quite a lot of potassium and as for iron it's is just a trace so as long as you have some. There is times when iron can convert to a non plant available form which isn't good for fish but I very much doubt that was linked to the fish deaths.
I use to dose C)2 injection and liquid carbon together which I am not sure is wrong or right and if the proportion are correct as I use 100% of max dose of each.
Nothing wrong with that.
I think the Iron dose was around 1 instead the 0.2 as per EI.
EI is 0.5ppm Iron per week so try and maintain that. 0.2 is the dose you are getting from your 162ml from TNC lite hence it suggesting 2 or 4 times so you get 0.2x3=0.6ppm weekly
The phosphate is 0 I am using Phosguard for long time and I stop using it now to see if any improvements.
Yeah stop using that, you need some Phosphate in your life or more importantly your plants do. The 5ppm readings you're getting a probably wrong and once you sort out your Nitrate issues the plants will soon chomp through the phosphate.
The aquarium is heavily stoked which I think produse enough N for the plants. Ammonia is always 0 . Perhaps I need to use TNC complete instead of THC light ?
My understanding of this is that it is nigh on impossible to be stocked so much and feed that heavy that in a moderately lit tank like yours is the waste products would be enough to feed your plants unless your fish were crammed in there nose to tail. Plus being heavy stocked also adds lots of organic waste matter to the tank as well as the N and P which is the food for algae so you will need to do larger water changes in a heavy stocked tank which also removes any n and p you salvaged from waste.
The lights are 3 x 100 W LED flood lights working on timer 10 hours a day.
I would knock an hour or so off the lighting if you can until you bring things back into balance.