That is true. Let's say there are 100 fish. If half the fish are zebra danios then there are 50 zebra danios fish. If there are 50 fish left, and there are 2 times as many catfish as there are fancy guppies, then 33.33 of them are catfish and 16.66 of them are fancy guppies. Since one third of 100 is 33.33, then 1/3 of the fish are catfish.