When asking for sugar to a waitress in a restaurant, you will say, “Can I have some sugar, please?” Or when you go with your kid to see a big aquarium, you will say, “Look! Those fish are so colorful!”

Now you notice that even though you are talking about many grains of sugar, you still call it sugar, not sugars. Here we are talking about uncountable nouns. Compared to potato or phone or tree, sugar is very hard to count. So when potato become potatoes, phone become phones, and tree become trees, sugar will stay sugar in its plural form. Some other examples of uncountable noun:

oil – knowledge – love – intelligence – milk – meat – music

Different case for fish, because of course we can count fish! This is one case of irregular nouns that do not change at all when made plural. Why? Honestly I don’t know :D 

But here are some other examples: 

sheep – moose – swine – buffalo – shrimp – trout – aircraft – watercraft – hovercraft – spacecraft