All I can say is this.
Stop.
He threatens your pets. He threatens to kill your pets if you do something wrong, or upset him. In essence he is making you do what he wants, say what he wants, because you don't want to upset him.
Your dating two people. One is a really nice guy who makes you feel awesome. The other guy is not someone you really like, but he comes with the deal, and that is a load of $hit. You shouldn't have to wonder whether or not you are going to come home to an empty tank because of something you said or did that pissed him off. It isn't going to go away, it won't get better if he isn't willing to acknowledge that he did something wrong. My mom was with a guy that was like this, she told me years later after we left him that he'd killed his ex-wifes dog because he was mad at her. He tried to kill my moms bird, and our ferrets and probably would have. Fear and paranoia isn't a pleasant thing. Wondering when he's going to move from just threatening to really doing something sucks, wondering when he's going to move onto you or someone you love is hell.
He's playing the game of 'I gave it to you and I can take it away.' This isn't something a healthy stable person does.
Do you really think he doesn' t know better? This is obviously the NOT the first time he bought the wrong type of fish for your tank, and now matter who many times you tell him he still does whatever he wants concerning getting fish. I'm sure he's heard you complain about store employees not knowing what they are talking about yet he listens. Once, twice even, are accidentle, more than that and he knows exactly what he is doing.
Dump him. Don't call, don't write, don't ask any mutual friends how he's doing or what they think he's going through. Don't give him what he wants, which is you coming to him and trying to make things work between you. That's what he wants, you crawling back to him. And while you don't see it this way, he does. Don't give him ammunition against you by wrecking his stuff, don't give him the time of day.
I hope your fish pull through, and I know you will find that you can find a better partner, but I seriously doubt he will ever find one better than you.