How you go there depends on where you live. For those from the East, there is a direct bus No. 25 that brings you to Serangoon North. The bus stop is just after Yio Chu Kang Rd Junction and beside a Mosque ( It's nearer than taking MRT whic goes a big round). For those from the West it's easier to take a bus from AMK MRT - Nos. 25, 132, 155, 74, 165 from the same side as MRT(refer to bus guide for the right bus-stop to alight, and then cross the overhead bridge). Taking a bus from Serangoon MRT will be more complex as you will need to find your way to the shop.
I just wonder why you want to go to Petmart as they do not have any marine fish. They do sell freshwater fish, reef supplements, fish food and equipments though.
There are many other pet shops and fish shops in that area. There is only one lfs selling marine fish. but look at the price first.
Happy shopping!