The two centre legs willl be significant weight bearing legs. I'm not sure what you mean by saying the load is primarily on the edge and corners. From a pressure perspective they are the most likely to fail unless the perspex is too thin, but the pressure is equal on every square inch. Likewise weight / load will bear through every weight bearing support proportionally.
In this instance those legs will be bearing at least 1/6th of the overall weight each, i.e. if your tank weighted 1200lbs then there would be at least 200lbs through each of the two central legs - if you put a set of scales under each foot they'd report this and you could add all six scales up tot get the total weight / load.