Welcome to the frog forum ! I believe Strider is right and that they are poison dart frogs! Recommend not buying from store and instead either ordering from a reputable online breeder or a reptile show. There are many different species and the different colors cannot be kept together. A 75 could house about 5-7 darts comfortably depending on species. Check out our caresheets and dart frog forum for more info. A picture of the frogs you saw will also be appreciated