A reason not to feed your fish lettuce

[Salmon King]

I t was years ago I used to feed my fish romain lettuce.I used to buy it from a very reputable store.I would wash it very thorough.The fish loved it .One day I put some in a alge clip when things went wrong.All my fish took only a bite or two and started ackting strange.They started gasping and darting around the tank/the cowfish had mucas coming out there skin.I contacted a marine biologist I know and asked him what to do.He told me to try and get them to eat as much food as possible It was tence few hours but everybody lived.I contacted the store I bought it at and told it passed all there reqierments.I/m sure glad I didn/t eat it..What was on that lettuce. Does anybody know if all brands of nori the same and is it 100 percent safe.