How to stop a behaviour
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At this age redirection is best, just a quick no we don't hit that, then take him to go play a game or read a book
It might be an idea to post in the 'For the love of pets' group for any ideas from other fish lovers with big fish tanks on how they may have tackled this issue at different ages. Bless him for being inquisitive but like you say poor fish being bashed x
Bless him, you can see why he is interested in them! You could try ‘playing with the fish’ so sitting with him on the floor and showing him how to gently move your finger on the glass and seeing if the fish follow your finger. And then when he next hits it, instead of saying ‘don’t hit’ etc, you can say ‘play gently’ or whatever and see if that helps.. hope you find a way to help him xx
i moved my fish tank in to another room where i can have the door shut x
Depending on how big it is, you could use a fireguard.
As repetitive and frustrating as it is, just keep telling him we don't hit the fish, and move him away. He's not going to get it now, but by 18 months he will start to understand. If you can restrict access to it then do so. And practice "gentle hands" when you can. Let him help feed them etc model how you should behave around it. When he gets too rough, say we have to be kind, I can't let you do that, and move away from it.
Some sort of barrier between him and the fish tank. I find like with the opening drawers and then putting locks on, once they can't do something they lose interest in trying.