Is he still eating?. Being 6 months in the tank and eating it is unlikely internal parasites, the consequences of internal parasitea would have shown up much earlier.
A diet dominant in krill is a killer, usually due a nutritional defiencency caused by thiaminese found to be high in krill, and many types of silversides. If you are using any other brand of silversides besides Hikara, its a goid chance they are rainbow smelt, and will also contain thiaminese.
Raising the mg level to about 1400ppm may help with digestion, and feed him something else besides krill. Shrimp, octopus, and squid are some good options: as well as chunks of salmon and tuna. If he is not taking food, after raising mg, try some live ghost shrimp.