Lower tanks removed for clarity.

Rubbermaid model 2550 50 gallon Roughtote storage bins are used for water drainage.  Each tub receives the water from 9 tanks above,  The three tanks in the lowest rack drain directly into them while the middle and upper rack of tanks drain via PVC plumbing.  The basins are interconnected by 2 inch PVC through bulkheads placed in their ends.  The basins are inhabited by a few koi which do a good job of keeping them fairly clean and a few angels that have found their way into them.