The owner of a new pet store wishes to display tropical fish in display tanks. The above table shows the species that cannot live together. What is the minimum number of different tanks needed to safely house all the fish?

To identify the minimum number of different tanks, we're gonna concentrate in a fish species, in this case can be the A: as you see in the table, the A species can live with all the fish excepting the F and G, by their side, the F and G can't live together , by this reason, this three species must live in a different tank, in the next form:

Tank 1: A
Tank 2: F
Tank 3: G

Now the B species, it can live with A, F and G, but for this example we can put in the tank 1 (the tank of the A species). The C especies can live with A, F and G, but how we have A and B together, we're gonna put the C especies in the tank 3 (the tank of the G especies). The D species can live with A and G, we're gonna put in the tank 1 because can live with B species too. The E species can live with A and F, we're gonna put in the tank 2 (the tank of the F species) because the E species can't live with D that is in the in the tank 1. Al last, the H species just can live with A, E, F, and H species, by this reason, the only tank that can be put is the tank 2. In this form, the order is the next:

Tank 1: A, B, D.
Tank 2: F, E, H.
Tank 3: G, C.

And the owner of the pet store must buy three different tanks to display these tropical fish.