12.5 gallons of epoxy sounds like a bit much for what you plan to build. I provided some input on your previous thread dealing with epoxy since I built a 16' cedar strip canoe using West System & fibreglass cloth over the cedar strips.
http://www.canreef.com/vbulletin/showthread.php?t=78068
I ordered the cedar, fiberglass cloth & epoxy from Great Northern Craft in North Van. Cedar was all 1/4 inch thick, don't recall the weight of the cloth. The Bear Mountain Boat Shop plans for the Bob's Special design I purchased at Lee Valley Tools in Ottawa just before I moved to Vernon.
Back to the amount of epoxy required to build my boat. I still have almost a half gallon of epoxy and just under a quart of hardener left over from this project. The original order came with 2 x 2 gallons of 105 B resin & 2 x 0.33 quarts of 207 SB hardener. Don't know anything about the epoxy brand you ordered but still sounds like way too much to me. IIRC I put down 3 or 4 coats of epoxy on the outside & inside of a 16' boat using just over 3 gallons of epoxy & maybe a quart of hardener. I'd say 6 gallons of epoxy would be more than enough for what you plan to build. Not sure what ratio of hardener that requires for the brand you ordered, but probably half or less.
I also ordered 6 oz of 403-6 microfibres. These can be added to the epoxy to make it thicker for use as a glue to bond pieces of wood together (clamped) or as a filler (depending on the amount of fibres used). You can also mix sawdust or small wood shavings into the epoxy for similar results. The West System comes in cans with appropriately sized pumps available to meter the epoxy & hardener in the correct ratio, so no need to figure out the ratio required for proper curing. Two squirts from the resin can requires two squirts from the hardener can. Different numbers of West System are for different applications and varying cure times.
As far as building itself goes, I'd say completely assemble the tank first with all the screws etc to hold it together. The fibreglass cloth can be cut & overlapped at the seams to ensure there are no folds along the flat surfaces. It does lend itself to some stretching and is relatively easy to work with. Any larger holes should be filled before the epoxy goes on, but smaller ones may be left alone since the epoxy will fill all the crevices. Using this approach, there should be no need to silicone the corners or perform additional waterproofing. Doesn't matter whether you do the outside or inside first, but finish one & allow to cure before doing the next. In my case, I completely finished the outside of the boat (all coats) & allowed to fully cure before flipping it over & doing the inside.
Applying the epoxy can be a bit nerve wracking since you need to proceed relatively quickly when working large surface areas and this is where the properties & cure time of the brand you're using becomes the driving factor. To build my boat I had a helper mix the epoxy in small margarine containers while I applied it. When I was part way through using the first container, I had my assistant start mixing the next batch. This covered about 3 to 4 feet of length. This procedure ensures that when you start applying epoxy to the next section, the first is still wet enough to blend into the new section you are starting. For the surface area you'll be dealing with, you might be able to do cover a complete side (inside or outside) in one shot. At some time during the process, you need to scrape off the excess epoxy with a plastic squeegee for a nice smooth surface. This also ensures the first coat is properly absorbed into the fibreglass cloth and there are no bubbles. Bubbles are to be avoided at all costs since these will weaken the structure. This is where the proper mixture ratio, ambient temperature and good quality epoxy pays dividends. You'll need a grunge container to take the excess epoxy you're scraping off. Empty frozen orange juice cans (the cardboard kind) with a slit cut into the side work well to clean off the plastic squeegee & catch the excess.
For additional info, any boat building sites are a valuable resource, since the goal is pretty much the same, a strong, waterproof structure. Here's a link to the Bear Mountain site. A book such as Canoe Craft or Kayak Craft is also a great resource for step by step instructions on applying epoxy. Don't forget a couple of coats of good quality marine varnish to protect the epoxy from breaking down. This applies more to UV from sunlight, but marine aquarium lighting tends to be pretty intense as well, so an ounce of prevention here can save you some grief down the road.
http://bearmountainboats.com/
Edit: FWIW, I don't think you need to use marine plywood since the epoxy & fibreglass will waterproof the whole thing anyhow. I used ordinary carpenters glue to bond the cedar strips together. Save some $$$s by using construction grade plywood.... unless looks are important, then use something that's 'good' on one side.
What the hey, here's a link to my windows live photos of a good portion of the boat build.
https://skydrive.live.com/redir.aspx...1B64909474!968