why do my ottos die???

I just bought my first oto yesterday :dance He is adorable (actually, I have no idea if he is a boy or a girl). He is in a tank with 9 live plants, 3 harlequin rasboras, 3 red zebra danios, and a male betta. I will be adding some pygmy corys soon. The tank has been running for one month. I would appreciate any advice on keeping him alive, too! I have no salt in my tank. I have offered him zucchini. I am growing algae on a rock on my windowsill for him. Any other suggestions??

I would appreciate any pygmy cory advice, too :) Thanks!
 
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