Ready for fish?

im feeding it flake formula 1, which he wont touch and frozen brine shrimp which is what the store employee recommended to me. he ate a couple little shrimp yesterday though.
should i add another, it is an ocillaris.
i have a 20L and want to add a couple more fish, but if i want happy clown fish and need 2 to do it, i will
 
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