Traphunter,
Couple of additional things like everybody else has given, it sure sounds like ammonia is killing them off, ammonia is heavier than water so it collects at the bottom of the tank, fishes gills absorb ammonia faster than they will O2 so they go belly up in a hurry if you can't get rid of it.
Suggest that you may go with a sprayer bar with a submerged pump like a Rule that will pull the water up off the bottom when the ammonia hits the air via the spray bar it will then disburse to atmosphere, that way you won't have to do a water change, I would also use a lot of salt but use pickling salt, it breaks down fast and has no iodine in it which is instant death to baitfish.
My rule of thumb is 3 baitfish per gallon of water, but I'm using a Grayline baittank which has a filtering system and sprays the water out so baitfish swim in a circular motion, I am keeping Shad alive in my tank and they are much more fragile than suckers and shiners, you can also use a product called Baitsaver which adds electrolites and slime coating to there gills and prevents them from absorbing ammonia.
If your water is getting dirty or starts to stink after awhile then you will need to change the water out, if you have a lot of foam on the water that too will prevent oxygen saturation of the water and they will go belly up on you to prevent foam from forming use Cremora coffee creamer in powder form, just sprinkle a wee bit till you see it disperse.
Ideal water tempature should be around 50-60 degrees, use a digital temp meter if you need one send me a pm and I'll send you one.
Something else, look at the water see if you see sparkly flakes in it, lots of times they will drop scales and when they breath those it cuts there gills and that is also a bad thing, again the pickling salt will lock down there scales preventing flakeoff.
Doc