IMHO, leave the market prawn in there, dun take it out. wait for ammonia to be present (by testing), wait for ammonia to be zero (test), wait for nitrite to climb (test), wait for nitrite to be zero (test), wait for nitrate to climb (test), switch on skimmer, do 20 to 30% water change and monitor, if nitrate still unspeakably high after a week or 2, do another water change.
juz me 2 cents of doing.......