All other tanks are primarily simply to add water volume for stability. The 60 gallon is simply a place the other tanks drain to. It contains a set of filter socks (however the 55 gallon refugium is plumbed past those socks so as to not filter possible creatures that grow there and give them a better chance to make it to the display). The 60 gallon also houses the return pump and catches all water in the case of a power outage.