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Hi @jsmit598,
Sounds like a wonderful project. We are looking forward to seeing it come together. Let me tag wonderful members @Brad, @gippslandhome, @Dale, @ProjectPete and @TheSaltyreefer who might like to share their experiences with you.
Let me also extend a very warm welcome to the Workshop community. We trust that you'll get plenty of helpful advice and inspiration for many projects around the house and garden from our amazing community members. Please feel free to post anytime you need help or have something to share with us.
Jason
Hi Mate
Firstly what size aquarium are you looking at and salt or fresh?, we were going to build our 12x4x2 Stingray tank out of ply or part there of, but was cheaper to build it out of glass.
I have had a few friends build them, some have used fiberglass, I have seen Epoxy, and I have seen various types of water proofing agents used.
Structural ply is ok for the build as long as the waterproofing agents can saturate into them deep enough, Marine ply if you want to go full on. For me if I was going to do it, I would F glass it.
I will get alittle more information for you after you let us know size etc 
Only other thing to look at also, if it is going to be a large tank make sure you look at where you are placing it, I have seen some large tanks come through floors etc, not placed on weight bearers
