Cellz.io is a fantastic Agar.io clone with alot of new features.
Your Goal in Cellz io is to grow the cell as big as possible and try to become the biggest one in the game. At the beginning accumulate the mass by absorbing color dots
As soon as you're big enough to absorb the smaller enemies go and do it. Also you can stay close to a big players, it's dangerous, but a quick way to receive a lot of mass.