Ethoxyquin was originally developed by Monsanto as an herbicide/pesticide in the 1950s. For years, studies regarding its safety were done or sponsored by Monsanto. Nope, no red flags there.
The
U.S. National Library of Medicine says the following on its page for ethoxyquin under Safety and Hazards:
Signal: Danger
GHS Hazard Statements
H302: Harmful if swallowed [Warning Acute toxicity, oral - Category 4]
H317: May cause an allergic skin reaction [Warning Sensitization, Skin - Category 1]
H320: Causes eye irritation [Warning Serious eye damage/eye irritation - Category 2B]
H370: Causes damage to organs [Danger Specific target organ toxicity, single exposure - Category 1]
H372: Causes damage to organs through prolonged or repeated exposure [Danger Specific target organ toxicity, repeated exposure - Category 1]
H400: Very toxic to aquatic life [Warning Hazardous to the aquatic environment, acute hazard - Category 1]
H410: Very toxic to aquatic life with long lasting effects [Warning Hazardous to the aquatic environment, long-term hazard - Category 1]
So any documented ethyoxquin-related fish deaths in the history of aquarium-keeping, where fish died after moving from a food with lower concentrations or none to one with higher?
This is relatively meaningless. First of all, you or anyone else here not being aware of such documented cases proves nothing as to whether they exist or not. It's probably safe to say no fish food has a high enough level to cause acute poisoning. It's highly unlikely someone has fed their fish a product containing EQ, then had the fish die so soon thereafter that they rushed right over to their local aquatic toxicology lab where they proved or disproved the EQ in the food killed it.
Secondly: The toxicity of EQ is not even a question. It kills stuff, including fish. Created as a pesticide, it was
intended to kill stuff. It's used as a preservative
because it kills stuff. In fact, some countries ban it as a pesticide. Just what it kills or how quickly depends on exposure and dosage. EQ has been demonstrated to accumulate in fish tissue (
study). It's been demonstrated to cause liver changes. It metabolizes into a brew of other compounds, some of which are suspected or have been demonstrated to have negative effects on fish (also humans).
A valid question, already raised, is how many fish have become sick where chronic or excessive EQ exposure was an undiagnosed cause of contributing factor? No one can really answer that one way or another. What I can answer is the causes of
most aquarium fish deaths are unrecognized, undiagnosed, or attributed to 'stress', which can mean a thousand different things. But, no
proof of anything here, one way OR the other.
So the real questions are what does long term exposure to dietary EQ do to fish and what should be the max levels in fish feed. The first question might be considered as debatable, or yet to be determined. As to the second, the FDA says max should be 150 ppm. CVM has reportedly requested a max of 75 PPM (
reference). So, no one should notice when a product is documented to far exceed these limits? No one should point out that it's deceptive to indicate your product has no EQ, when it has twice the FDA max?
If that doesn't bother you, based on however you want to reason on it, you have a right to your opinion and a right to make your case. What's invalid is to simply dismiss the other side of the argument or those who feel it merits some concern.
https://www.hindawi.com/journals/ijfs/2013/585931/#B28
Ethoxyquin has been used as an antioxidant in animal feed for several decades and despite the search for new compounds that could be used as free radical scavengers, it is still the most effective antioxidant. The negative health effects in domestic animals fed with EQ containing feed were observed some years ago, but the presence of its approved doses should not be hazardous. Toxicity and mutagenicity of EQ were observed in in vivo and in vitro studies showing its potential harmful effects. This makes it very important to label all products and ingredients to which EQ is added and to comply with the recommended doses. Additionally, the results of the studies on products of EQ oxidation, especially EQDM, detected in farmed animal tissues indicate that it should be under control and some regulations should be introduced.
Keep in mind that what is toxic for one critter is not toxic for another, per se. 
