My fish bowl smells really bad. What can I do?

I recently got a male beta fish. I've got him in one of those 'fish bowl' margarita glasses with glass rocks, seashells, and an aquatic plant. I clean it once a week and put in a water conditioning fizz tab. The smell is driving me insane! am I doing something wrong? what can I do?
any immediate fixes would be appreciated too
 
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