Sounds ambitious, be sure to post pics!
Either all 2x4's with verticals doubled up or 4x4 uprights notched to accept the horizonal 2x4's. One vertical between each tank and some diagonal cross-bracing should do it. Can you anchor it to a wall? Sheeting it with plywood would be benificial too, but with proper bracing, not necessary.
Have you looked at some of the fish suppliers? Some show pictures of their stock and breeding tanks and even shipping areas. Somethimes you can glean alot of info from the pictures. Check some lfs's too.
Do you have a room long enough for this?
Good Luck,
Dr Joe
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