Mincemeat is usually made up of all the off-cuts of meat and includes the fat etc. also unscrupulous butchers will add preservatives (they are not supposed to but they do) believe me I know from being a butchers apprentice many moons ago.
I feed beef heart to my Killifish (now and again, mabe once a week at the most).
I buy a beef heart and cut the leanest meat into cubes and discard the rest. I then freeze the cubes. Next I grate the frozen beef cubes with a hand grater through the smallest holes. The meat now has the appearance of red worms which my killi's love. But as said earlier, feed sparingly and do not leave any uneaten food in the tank.