The amount of room in your reef will change each time you level up in Splash. You can see how many fish, jellyfish, sharks etc. you are able to have on the Aquapedia button in the top right hand side of the screen the number will show the current number/how many you can have e.g. 23/40
Currently, the maximum reef level is 48 and maximum total of species in a reef of this size is 63.