Each level in Osmo Numbers has three types of fish. In order to unlock the 3rd type of fish and get the highest score possible, you will want to strategize in order to slow down how much water falls into the tank and play for longer. This is primarily achieved by getting combos.
Combos are earned when you pop at least 2 or more water bubbles with a single move. For example, if there are two 6's, then put down two tiles that equal 6 (e.g., 2 + 4). This way both 6's will pop simultaneously giving you bonus points AND will evaporate so the tank won't fill up so fast. Try to strategize such that more than one of the same number becomes active before you target it (when possible). Another example: If there are three 10's, then place 3 tiles that equal 10 (e.g., 5 + 3 + 2). This will result in a triple combo!