not to sure what the popocy is in putting up links to other sites on here,
however just google "plywood fish tank" and there are a bunch of links that come up when i did it the first 2 had plans
And just a word of advice... Don't expect the construction of a plywood tank to be fast . Properly joining the wood and then properly sealing it up with enough epoxy to prevent leaks is a long painstaking process.