Business owner Michael Smith says some of his animals died from excessive heat, while others may have drowned when the sprinkler system came on and flooded their tanks.
The store sells a variety of animals, such as hamsters and birds.
Smith estimated at least 200 animals were dead on the scene, or will not recover. Hundreds of others survived.
Investigators are trying to determine if the fire was electrical. Smith says heat lamps may have contributed to the blaze.
McKinney is 30 miles north of Dallas.
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