setting up 7 500 gallon tanks need help?

I plan on wholesaling minnows. I have a few ideas on how to set up my tanks but I would like to make sure. For my tanks I was thinking of using half of septic tanks which are about 500 hundred gals. each. I can't use a flow throw system because of water so i need a good filter system. I will only be able to add about 2000 to 2400 gals. of freash water a day. Now i've seen complete filter systems for sale but I thought I could build one cheaper than $4100. (flow rate 60 gal/min. filter 2.2(ft3) uv 120 watts) should take care of 5000 gal system with 120 to 150 lbs. of minnows. What type of media mixed media, beads or sand? I've heard sand sometimes cakes? Any help would be helpful. Thank you. Wijohnsen