The b factor in the standard allometric equation W=aL^b is really determined empirically by collecting a number of fish either under various conditions you describe or alternatively from pooled data to give an overall average.  Fulton's K on the other hand simply assumes that the weight of the fish is simply proportional to the cube of the length K=100(W/L^3), so in Fulton's equation the b exponent is a constant.   Fulton's CF is often useful if you want to get a "quick and dirty" comparison between fish populations when the empirical study to determine b is not done.  The allometric equation becomes a better measure in flatfish such as flounders and soles in which the b exponent can deviate substantially from the approximate estimate of 3.

Indeed - using measures of condition like relative weight are preferred as K should be limited to comparing fish of the same lengths since it assumes isometric growth.  Some nice reviews attached...

Fulton's K  assumes that the weight of the fish is simply proportional to L3  and in the equation the b exponent is a constant.  Fulton's CF is useful for comparison between fish populations when the empirical study to determine b is not done.The b factor in the standard allometric equation  is determined empirically from the pooled data to give an average value.  

can I use this condition factor for cephalopods which shows allometric growth?

Hi Jasmin, as explained earlier, the Fulton's condition factor assumes b = 3. If you have species with allometric growth I would recommend the relative condition factor (Kn) proposed by LeCren (1951), which is not influenced by length. Cheers.

Good growth condition of the fish is deduced when Kn ≥ 1, while the organism is in poor growth condition compared to an average individual with the same length when Kn