I agree with Boredomb on the dual light periods as I will explain, but first to answer your initial question
there is a combination of issues affecting the Echinodorus.
First is light, it is in shade and that is going to seriously affect the growth of red-leaf plants which need more light intensity than green-leaf plants. This is because red and blue are the light colour plants must have to photosynthesize, and the leaf colour is reflected light, so red leaf plants are reflecting red light and thus need even more of it. The intensity overall has been proven to provide this, rather than adding red tubes or something. So first I would try to get the floating Salvinia balanced throughout the tank. There should be some over the open area to protect the fish, something i will come back to when I discuss the light question.
Second, the nutrients. CO2 is as you have surmised (with your dual photo-period) likely the nutrient being depleted first, as it is in most natural setups. Encouraging it by not touching the substrate helps. The other nutrients may or may not be lacking, depending upon the substrate soil and fish foods. All the necessary nutrients will occur from fish food, but obviously not in great quantity. My tanks are fairly heavily stocked, and I know from 15 years experience that the fish food does not provide sufficient nutrients without some form of fertilization. The easiest and usually most sufficient is a complete balanced product like Flourish Comprehensive Supplement or Brightwell Aquatics' FlorinMulti, either dosed once a week.
And this brings me to the balance, the all-important issue in planted tanks. Light must balance nutrients. At the risk of also being criticized for commenting without all the facts, since I don't know the length of the four T8 tubes, I would suggest from the appearance in the photos that they are 48-inch; the light reflected in the tank is very bright. This is double the intensity needed to balance your nutrients. And no matter what photo periods, this is not going to work. Your comment on algae occurring supports this--if the light is balanced with nutrients, there willnever be algae issues in a planted tank; these only occur when the balance is somehow out. The dual photo-period addresses the CO2 issue but not the others if any of them are lacking. Plants will photosynthesize to the max with sufficient light intensity--provided all 17 nutrients needed are available. As I've noted above, this is not likely to be occurring, or to perhaps put it more correctly, the light is greater intensity than the plants can possibly use because the nutrients are limiting them. If I have correctly assumed 48-inch tubes, removing two of them is strongly recommended. This plus adjusting the floating plants would almost certainly change things.
Now to the dual photo-period. I appreciate that this was not the question asked, but please understand that this is a forum for the exchange of ideas and advice, and when any member sees something mentioned that he/she feels is likely to cause problems, it makes no sense to ignore it. We are all free to accept or reject this or that view, fine. But there are issues with light and they should be recognized.
Walstad and Barr are plant people, not fish people, and this should be borne in mind. Some time back I raised this very issue of a dual light period on one of their forums, and there was no scientific evidence provided to refute my views. Light has an impact on fish, significantly. Forest fish occur in very dimly-lit waters, few of which are ever exposed to direct sunlight; and if they are, the fish invariably remain under branches, floating vegetation or overhanging terrestrial vegetation. There is good reason for this, but rather than repeat what is written in my article on light, I will post the link; please have a read of it, it may help to explain this:
The specific issue of dual photo-period is not covered, so I will briefly comment. The circadian rhythm described in the article is internal in all animals. The stress of going through two periods of day during 24 hours cannot logically be denied. Fish are captive to the conditions we provide in the aquarium, and any form of departure from the norm will affect them, because they cannot escape it. I go into this more in my article on stress, so I'll end with that link:
I hope this may have helped to explain things.