Sunflower question

Hi. I bought sunflower seeds from a pet store. They were the tall varieties because the seeds are long and have black and white stripes. The seeds look fresh and good. But when I conducted a seed germination test, the seed coat split open and it smells like rotten fish. I used 20 seeds and only 5 germinated. The first time I bought the seeds in the same store, it all germinated. But my question is, how to know the seed viability of the sunflower seeds? And also, why did my sunflowers became dwarfed if the seeds are the tall variety?

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