[math-fun] Square hex numbers?

Just happened to notice that in the centered hexagonal numbers H_n = 1 + 6 T_n = 3n^2 + 3n + 1 we have H_7 = 13^2. Also H_0 = 1^2. So I tried to find all such cases of H_n = K^2, but am not sure of the best way to proceed. Suggestions? —Dan