How to prevent back flow for parallel connected tanks?
I would use a non-return valve




  Diameter: 0.75 in
  Maximum Bar Pressure: 16 bar
  Type: Non-Return
  Colour: Black-Grey
  Material: PVC   
  
  Ideally used for conveying water and other fluids in urban and inter-urban infrastructure, industrial, mining, landscape, and farming applications


Note: This one seems expensive, I suspect I can buy about ten 22mm metal-bodied double-check valves for this price.

The secret to buying plumbing parts is to specify exactly what sort of pipe you are joining, materials, nominal and actual diameters, joining method, types of fluids and pressures to be carried and so on.

It is disappointing when you find your 40mm push-fit spigot won't fit into your 40mm "Multifit" compression coupling, or you find that your spare parts box has 40mm ABS solvent-weld elbows but only 40mm  MUPVC solvent-weld pipe (and your PVC-only solvent has jelled anyway). Find a local plumbing supplies outlet and try to build up a good relationship with them so you can get advice and can return parts that don't fit the way you expected.