SOUNDS LIKE THE FISH LOUSE, MANY CAN JUST BE REMOVED FROM THE TANK BY HAND,ONES ON THE FISH CAN BE REMOVED BY RUBBING OFF( GOING FROM HEAD TO TAIL ) OR WITH FORCEPS. GET SOME MALACHITE GREEN AND USE AS DIRECTED TO HELP KILL THE REMAINING BUGS AND THEIR EGGS.FOR THE DEMPSEY THE MALACHITE MIGHT WORK ALSO BECAUSE IT PROBABLY HAS A PARASITE TOO, YOU COULD ALSO TRY A SALT BATH ( ABOUT A 2% SALT SOLUTION PER GALLON ) IN A SEPARATE TANK FOR ABOUT 15 TO 30 MINS (KEEP A REAL CLOSE EYE ON THE FISH DURING THE BATH ) YOU CAN REPEAT AT ABOUT 2 DAY INTERVALS FOR UP TO FOUR BATHS FOR SEVERE CASES. IT'S HARD TO SAY IF IT WAS THE FOOD BUT SINCE IT IS IN BOTH TANKS IT IS POSSIBLE ,MOST MARKETED FOOD HOWEVER IS USUALLY SAFE TO USE