400 gallon bait tank ?????????

got a 400 gallon i keep gills in at home? lost about a couple of them a day and was wondering is there anything else i can do to stop this? i use one of the bucket filters i read about on here with, lava rock poly filler foam filter. the water is crystal clear. keep some salt in tank also. also change water out about every 5 days, 20 percent change or so. been checking chem in tank and sometimes nitrites are higher some days but i think it may from dead fish, i try to get them out quick as possible. is there any other additives or filters i could get to control nitrites and keep my fish living? thanks for all the help, love reading post on here and getting all kinds of new ideas