craigslist for a 75gallon for like 50bucks, and aren't you planning on moving or your grandparents don't want you to have more fish...?!?!? your threads are really contradicting....
I finally reparied a plywood tank 500 gallon. it was org finished with fiberglass and it has leaked after a few years so the ower left it unused for years. I tried two different kinds of expoy paint they only worked for a few days then the water started to cloud badly which ment that water was getting to the wood. I then tried ugy water proof paint it suppose to hold back 27 feet of water it to leaked through after a day or two.
I finally found ames blue max liquied rubber it worked!! the tank has been up for a month no problems blue max is 800% fexiable that was the problem with the expoy pain it is very brittle and shift and you have a leak. good luck I wish I know about blue max from the start it would have saved me a could of hundard dollars