This is a very good example of why you cannot trust most of what you find on YouTube.
I listened/watched this video for five minutes, and the amount of inaccurate information was astounding. I do not have sufficient time left in my life to even bother with someone like this. It might help him to get some education in botany. But it is certainly a black mark against Aquarium Co-op if they endorse this individual [and it is not the only black mark, that's another whole issue, but at least it is consistent with other issues].
Flourish Comprehensive Supplement for the Planted Aquarium...he is correct that not all nutrients are present, but he does not seem to have the intelligence to understand why this is good, not bad. The only three nutrients (of the 17 that plants need) not in FCS is carbon, oxygen and hydrogen. Now, why would anyone even remotely think oxygen and hydrogen need to be added in this liquid form to an aquarium containing oxygenated water? That is just absurd in the extreme. I am not aware of any fertilizer containing oxygen or hydrogen as nutrients. As for carbon, it is more than amply available in an aquarium with fish that are being fed. It may be limited, but that brings us to another of his misunderstandings...for what these products are intended.
FCS is--as the name should suggest to anyone with half an ounce of intelligence--a supplement, not the source, of nutrients. Granted some of the micro-nutrients may be available only through such an additive, but FCS is not intended to be a source of macro-nutrients but a supplement. And there are a couple of reasons why this is significant. But first it may help to list the 17 nutrients.
Macro-nutrients: calcium, carbon, hydrogen, magnesium, nitrogen, oxygen, phosphorus, potassium, and sulphur.
Micro-nutrients: boron, chlorine, iron, nickel, copper, manganese, molybdenum, and zinc.
The macro's in most any quality comprehensive supplement will partially be supplied from other sources. I have already dealt with oxygen, hydrogen and carbon. Calcium and magnesium (the primary minerals in GH) is often found in source water so these are replenished with water changes; it is a bit different for those of us with very soft water, and additional supplementation of these two may be necessary depending upon the fish load/feeding and plant species.
Nitrogen...only a total fool would want to add nitrogen to a tank with fish. Most species of aquatic plants we keep use ammonia/ammonium as their source of nitrogen, and they can assimilate a lot of it day and night. They are faster at this than the nitrifying bacteria (the "silent cycle" aspect). And any tank with fish being fed is not going to be short of ammonia/ammonium insufficient for the plants in a natural or low-tech system [high-tech is admittedly a very different ballgame, outside our scope here]. Fast growing plants especially floating are not termed "ammonia sinks" for no reason.
As for phosphorus, there is more than enough for plants in fish foods. It should never be added (again, talking natural or low-tech here, which is what FCS is dealing with).
BrightwellAquatics' FlorinMulti is basically the same as FCS. Another identical product is The Nutrient Company's TNC Lite (this has no nitrogen and phosphorus) or their TNC Complete (has nitrogen and phosphorus, but minimally like the other two); TNC is only available in the UK so far as I know. There may well be other similar comprehensive supplement products just as good, but these three/four are top of the line. Assuming you use them as they are intended to be used.
I didn't watch any more, there is no point; I had more than enough to discredit and discount this fellow.