His heter was robust, but it was not on his list of shitos that made him not hold of a person who argues (the eruv and worms in the fish are on that list)
Not robust at all. (They are visible without a black background).
Famously, when asked about the water, he responded that he held it was muttar, but others (referring to R’ Dovid Feinstein zt”l) asserted. Interesting that you say that the fish issue was a passeling issue since R’ Dovid was machmir… The eruv issue is based on a machlokes acharonim, so on the actual shaila you can’t passel someone for being meikil, but perhaps since in NYC R’ Moshe’s psak was accepted, he felt it’s מקום שנהגו בו איסור.