The best place to look for palatability enhancers is their natural habitat and a good observation! Each species, each environment has typical molecules. Your observation may yield guidance to others. good luck. Try where your fish live naturally.

For marine shrimps,shark liver oil is one of the best attractant.10 ml of it is added to a Kg. of the raw dough before making out the feed pellets, which are then dried.For fast flowing riverine fishes,one of the scientist used Asafoetida in fish feed & through hooking,he got good results.

Depends upon which species of fish you are working with; certain free amino acids and amino acid mixtures work well for some species (particularly carnivores), betaine-glycine and nucleosides and biogenic amines at low concentrations may also have a positive effect for some species 

I agree with my friends. It depends on environment and species of fish. Fishmeal (FM); blood meal (BM), meat and bone meal (MBM), squid meal (SM), fish oil (FO); fish solubles (FS); potato protein concentrates (PPCs); squid meal; krill meal and methanol (MeOH) extract of mackerel are some kind of attractants which are using in fish diets.

I'm agree with my friends. It depends on fish environment and species. 

Thank you every body for your answers, i will take into account

try krill, fish hydrolysates, individual amino acids: betaine, glycine, alanine;

It is completely depends on fish species. Fish that find their food by olfactory and gustatory system, such as sturgeons, catfish and common carp, has great response to certain attractant, especially dispensable amino acids. There are many literature on this topic. Be careful, if the basal diet contains high levels of fish meal, usually no needs for attractants. You should apply attractants in non-fish meal based diets such as soy-, soy concentrate-, meat meal-, gluten- and casein-based diets.

Good luck