Almost all seafish is toxic even in small, weekly amounts. I think the main contributor to this is mercury, that was once used in industrial processes, which basically still all sits on the bottom of the sea (where many fish eat from).
What about the potential mineral leeching issue with RO? I use a RO filter and mix it with 30% glass bottled water, both for taste and in hopes of avoiding any potential issue with leeching. I searched a bit but found nothing useful on the topic.
The theory seems to be that the lack of minerals in RO water might serve to leech minerals from one's body. I think there's also some concern RO water might leech from cooking vessels and organic compounds in plastic tubing, etc.
I have no idea how real the risk is, and it all seems less risky than drinking unfiltered water in most areas.
But it's no headache for me to mix RO water with bottled spring water, since I prefer the flavor and it takes seconds.