Maybe father fish scouts the internet for info such as this, and then regurgitates it on his channel.....
What is a safe nitrate level in an aquarium?
aquariumscience.org
Interesting comparison here, and from a site that often has a lot of interesting and useful info...but not necessarily contradictory to the "common knowledge" used by many aquarists.
That study considers nitrate toxicity based upon the levels required to literally kill fish in 72 hours. Now, admittedly, there are posts seen regularly here on MFK and elsewhere that might make one think that some people are indeed lucky if they can keep their fish alive for longer than that...but it's safe to say that most aquarists are hoping for a bit more longevity.
When long-established aquarists like
duanes
refer to the toxicity of nitrate, they are typically considering the toxic effect over a period of years. The commonly quoted effect of nitrates, especially on soft-water fish that are being kept in hard-water that is also nitrate-rich, is HITH...and that is indeed a problem that crops up only after an extended period of exposure. Plenty of smaller aquarium fish won't even have a natural lifespan long enough to develop those symptoms.
Evidence? There may not be any, if you define "evidence" to mean quantifiable results of rigorous scientific testing under controlled conditions in a lab. Science costs money; if you want to maintain multiple tanks of Oscars in a lab over a period of years, you might very well be able to provide this type of evidence...but for sure you would require extensive long-term funding that would be provided by...whom, exactly?
But when numerous aquarists make observations on private aquaria over such a period and come to similar conclusions...this used to be referred to as "citizen science"...I think it's still evidence, albeit not published in journals or adding to someone's credentials in the scientific community. The "real scientists" like to denigrate this by referring to it as "anecdotal" evidence.
I don't think Father Fish is much of a real scientist or a citizen scientist. He's just another guy with a YouTube channel out to make a buck, who has found a niche that works for him.