Alberto puts 3 kinds of fish in his fish tank: zebra danios, fancy guppies, and cat fish. half of alberto's fish are zebra danios. he has 2 times as many catfish as he has fancy guppies.

Missing info:
Alberto's friend Jackson says 1/3 of Alberto's fish are catfish. Is Jackson correct? 

Given:
zebra danios
catfish
fancy guppies

zebra danios is 1/2 of the total number of fish
2 times as many catfish as fancy guppies

x = guppies
2x = catfish

x + 2x = 1/2 of the fish population
3x = 1/2 of the fish population
3x * 2 = 6x 
x = 1/6 number of fancy guppies

catfish:
2x = 2(1/6) = 2/6 = 1/3 catfish.

Jackson is correct. Catfish is 1/3 of the total fish population.