Uhg, kids. I just don't like them. It's not they they are uncaring or insensitive (though it IS often the case), but they are just... well... not smart. They have the mental capacity of kids. In fact, I think sometimes the 'stupid' things kids do are based in good. At 2 years old, they don't know what it is to be cruel yet. If they do something, it is because they think it should be done... Like yesterday when I was babysitting, my five year old cousin kept trying to feed their betta I gave them from Nashnut, "Mr. Peanut". I told her "I know you just want to feed him, but if you feed him too much, he will get sick. He has already eaten today." At that, she completly left the fish alone... However, when my aunt tells her "Hayley, don't feed the fish" she keeps on trying. With little kids, I think they normally DO have either good intensions or no intensions... And when they do something that results in harm, it is because they just did not know.
When you get eight and nine year olds killing your fish, then you need to worry that there is something wrong with them
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I think you have complete right to be annoyed at the mother, even if you are not. I think next time that kid comes over, you ought to before hand (since obviously this is a reoccuring sitation) explain to him nicely that the fish are very happy, and putting anything in their tank might make them upset or sad or sick... With a lot of children, I think getting that point across actually does a good job of keeping them out of trouble, rather than just yelling at them.
Erf... Sorry, that was my 17-year-old-girl parent talk >_>