Additionally, people have been killed in clinical pre/probiotics studies.
People with
impaired immune systems may be killed by almost every kind of microoganism, even by almost ubiquitous molds and other fungi.
Please show me a proof that people have been killed by prebiotics. This is completely new to me, and we are discussing only prebiotics here, no addition of living bacteria. To my knowledge, the positive effect of fibre and prebiotics on intestinal health and general health is not doubted, in contrast to probiotics.
Please remember, you blamed me for obvious assumptions! What are you doing?
And bacteria cultivation (mono culture bioreactor?) is not a reef tank.
I don't understand this question, and translation by translator doesn't make it more understandable to me. What do you mean? Reef tanks are no mono culture, and also slow growing bacteria and other microorganisms have their chance in a reef tank, in contrast to bacteria cultures for economic profits.
Again, patients have died in probiotics studies, because a beneficial effect was assumed that turned out differently. A beneficial effect of a pre or probiotic in a certain environment cannot be directly translated to another environment.
See above.
There is a lot we don’t know in this hobby. But this happens to have an enormous amount of consistent peer reviewed scientific evidence behind it. So, yes, I think we can say “we know”.
No, you haven't read this peer reviewed scientific literature thoroughly, possibly you only know it from hearsay. Otherwise you should have noticed that scientific investigations in the effects of DOC are nearly always done with glucose and sometimes in ridiculously high doses that are suitable to consume most of the oxygen in the water, which alone can cause coral bleaching. You should also read and think about the "material and methods" chapter.
Like Randy and me have already statet, there are big differences between different organics for organic carbon dosing. I have run trials for many months. But we are not discussing DOC here.
Is more fibre always better?
Of couse not, but Reef Actif comes with an instruction.
Are all tanks always lacking fibre?
If not, how do I know that my tank is lacking fibre or not?
Likely not all tanks are lacking fibre but many tanks may.
If the tank is possibly not lacking fibre, adding a little bit more will not do any harm, only the benefit may be reduced or lacking. There are also different fibres which have different effects. We use a mix.
Algae eating organisms and algae eating fish that are fed seaweed also add those substances.
Could increasing algae based foods in our fish’ diets provide the same benefits to the tank as Reef Actif and at the same time improve our fish’ their gut health?
Feeding seaweed may have the benefits you describe, especially the health effect to algivorous and detrivorous fish, if feeding the right kinds of algae. The fish intestine of these fish is working as a fermenter producing short chain fatty acids which have health benefits for fish also.
I have no direct comparison if both, feeding seaweed and dosing Reef Actif have exactly the same effect, but yes, maybe both are alternative ways. I never have excluded this. By the way: I also have thought about the beneficial effects of Reef Actif described by fish enthusiast with reef tanks. Yes, maybe both methods are interchangeable.
It made my whole office stink for a few hours after dosing it 
