What did your water tests say at the time?
What about Salt, did you by chance salt the system for any reason?
If your neighbor was spraying pesticide, I'm very glad your fish are all ok but most pesticides don't kill plants. However if they were spraying herbicides that might kill plants but again, I'm happy your fish survived since herbicides can also do in if it gets into the water heavily enough.
Now depending on you location, heat can do in lettuce and watercress though I would have expected beans to survive heat. There are also plant diseases that can spread quickly in very densely planted gardens and this tends to be more of a problem in humid climates.