I have been keeping minnows in my garage since November. I started out with 3 lbs. and only lost a few. Last week I was about out so I put in 5 lbs. I have lost probably 100 or more in that time. I filter my water with a Homeade pump I bought from Sacolait. It filters the water through charcaol pellets and batting material. I too am wondering what is going on to kill so many. I have a 150 gal tank and I am running enough air pumps for 300 gal as well as running my discharge from my filter pump through a spray bar.