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Cold clean water. Sounds like you got the clean part down. In my experience the colder the better as long as their not froze
I add a few hand fulls of snow to the water & turn on the aerator to keep them overnight in the basement next to an outside wall.
How many pounds of bait are going into 75 gallon tank?You may need to make a filter or buy a very large one. Depending on how much bait you are keeping, your gonna need to filter alot of stuff out of water. More fish, more piss, crap that needs to be filtered out. I kept about 7 pounds of bait in a 55 gallon drum in my 63 degree-ish basement all last season. I made a filter system. It worked pretty well. I had Rosie's all the way up to pike shiners in the drum. Cycle the filter. As much as you can. I started with like a dozen shiners to star the filter cycling. Waited almost a month before I dumped my bait in. I still lost some fish but i think the filter wasnt cycled all the way. Just way too many fish at once. Couldn't filter out the junk.
My bait tank is only a 20gal. I plan on running a Rena XP3 filter on it. I'd like to keep a couple dozen crappie minnows a about the sms for roaches. I've got extra filter media cycling in the wet/dry sump on my 75gal now. Once we finally have some decent ice I'll set up the 20 with the filter media and about 15 gallons of water from the 75.