For what it's worth... I don't have much experience makin plywood tanks, but I have been building boats (up to about 50') with the stuff.
So, my advice:
Don't know about the recommendations. They probably apply, but it should be fine to either go for thicker plywood or make some kind of support for it. Make sure to have thick enough glass though...
About the tank itself. Some people are saying use marine grade plywood and waterproof glue. That is all good, but not really necessary. The whole point here is to use the epoxy to make a waterproof layer on top of the plywood. If this doesn't succeed, the tank is f****d anyway. Doesn't make any difference if it's "marine grade" or not. Same with the glue. However, you might as well use waterproof glue, the difference in price is insignificant.
About the epoxy. Make sure you get a good brand. I don't know who makes good epoxy, but I know that the "west system" will work. Many stores will say they have epoxy, but they don't know what they're talking about. If you can't get west systems, then get something else used for cold-moulding and strip-planking boats.
Working with epoxy sucks. It's poisonous, promotes cancer and whatever. You want to get dosing pumps to mix the glue and the hardener. They're cheap, and it makes things soooo much easier.
You want to make sure, after glueing the tank together, that there is no cracks or unevenness anywhere. If it is (and it usually will be), get some powder to mix mith the epoxy to make it thicker. Fill in everything. Sand down (wear mask). Make sure everything is perfectly smooth before starting "painting" with the epoxy.
Decide on your background. You get colored powder to mix with the epoxy, but the choises are limited. But you get black, which is what most people want. You can also paint the wood before applying epoxy, but then the the epoxy won't stick as good. Go for the powder if you can. But only on the last layer of epoxy.
You want at least four layers of epoxy. Thin layers, because it runs easily. After applying the second layer, you might want to add some of the aforementioned powder, to make it thicker. Use your own good judgement. And you should sand down the epoxy after each layer. You want to do this properly...
After everything is cured, you can silicone the glass in the front. Silicone sticks very good to the epoxy, so no problems here. I would recommend also to silicone all the inside edges of the tank. This shouldn't be necessary (if everything else is done properly), but it will let you sleep better at night
I don't know about the cost of this. I think it will be cheaper than an all-glass tank, and I know
it is a lot more satisfactorly. It is also a lot more sturdy than just glass. If everything is done right, it should last practically forever. And less chance of cracking anything.
Some people are worried about scratching the paint (epoxy), and recommend acrylic on the bottom and sides of the tank. I think this is silly. It takes a lot to scratch the paint. As I said, it can take a lot more beating than glass.
Anyways, you probably knew most of this from before... Good luck with the tank, and keep us posted