Well, for starters... the half-barrel that you found them in was
probably only temporarily flooded with rainwater. Nasturtiums DO NOT
like to have consistently wet feet, they tend to rot if this is the
case. As for whether or not they are safe for fish to eat? I'm not a
scientist or nutritionist, but I've eaten them myself in salads, and
as far as I know, they are safe for most mammals.... maybe do a google
web search on this question.