It seems to be alot of work for the amount of minnows that you may spawn. The addition of bacteria was not needed as the minnows and air movement will supply the bacteria or just a handful of dirt. If there is any structure in the 2,400 gallon tank the fatheads will try to spawn on it upside down. I think that the biofilter should be a minimum of a 55 gallon barrel and the water to enter the barrel on the bottom and flow upwards over the media. Depending on the amount of minnows that are in the tank after the spawn a second biofilter might be needed to act as a polishing filter. I am not attempting to hijack your hopes and desires, but I am in the wastewater consulting business and assist people treating millions of gallons of sewage.