New to DIY tanks. Need help

I have never used those products when it comes to sealing a tank but as for your question regarding the plywood thickness, 3/4" is fine. Make sure you double up on the top and bottom 2x4s so you don't get any bowing.

There's no purpose of putting fiberglass on the outside.

I'd suggest you look at JohnG's builds for the most current best practices. Specifically http://www.monsterfishkeepers.com/f...15713-7-x12-1400-Gallon-Plywood-Stingray-Pond his second build. His first can be found here http://www.monsterfishkeepers.com/f...4202-10-x14-2500-Gallon-Plywood-Stingray-Pond.

VLDesign's 2600 gallon build http://www.monsterfishkeepers.com/f...16005-My-new-2600-gallon-plywood-tank-project for his latest and see his signature for his 1500 build and the issues he had.

And of course, you can see my build and the issues I've had in my sig link.

Max ACR with fiberglass in the corners on the inside is the way to go.

That's the first I've heard of anyone suggesting fiberglassing the outside. I honestly think it is a bit pointless to fiberglass the outside. Fiberglass braced with framing from the outside is going to have far less give.

You fiberglass the outside of boats because the water is on the outside. You fiberglass the inside of bathtubs, jacuzzis, aquaculture pools, etc. because the water is on the inside.

Fibreglassing the outside will aid structural Strength considerably but only if you cover the complete outside. Doing just the external seems and corners will do very little. It will in no way help to seal your tank. If you are going to cover corner and seems only, build your tank to be structurally sound with timber alone and use it on the inside for sealing purposes only. Personally, I would use fibreglass reinforcement on all inside surfaces for better impact and abrasion resistance. I honestly don't understand why people don't use it more often. It really doesn't add that much to the overall costs and gives so many gains in return.