On to the waterproofing:
Because the tank has minimal bracing I decided to use fiberglass to reinforce the structure of the submerged section. I'd never worked with fiberglass before so, as with everything else about this build, it was a learning experience and I got better as a I went along. Here's how I went about it once I had the whole process figured out.
First I tilted the tank at an angle so that all residual epoxy resin would pool into the seams, deeply penetrating them and effectively creating a fillet. This was a little cumbersome because I had to reposition the tank for every seam, but it worked out very well.
I coated the seams with an initial layer of epoxy to saturate the wood and provide an initial barrier coating. I also dripped a little extra epoxy into the seams to make a slightly thicker fillet.
After this first layer had dried and was no longer tacky, but not completely cured, I layed out a strip of fiberglass cloth into the corner. I just used the cheap, lightweight Elmer's brand cloth from Lowes since I figured it would be adequate for my purposes. The lighweight cloth is pretty easy to work with. I found it made things easier if I took my time to make sure it was cut straight before starting.
Here's the strip wetted out with epoxy. After brushing it on I used the flat end of a stir stick and a gloved finger to really push it into the seams and force out any air bubbles. Make sure to work out the air bubbles while it's wet and you still can. Then I dripped a little extra epoxy on to really get a nice thick layer in the seams.
After waiting a few hours for the epoxy to gel (but not harden) I used my trusty paring knife to trim off the excess cloth to get a nice clean edges. I found that you shouldn't try to trim the cloth before it sets up or you'll pull it out of place and introduce air bubbles. Similarly, if you wait until it's completely cured it becomes too hard and sharp, making it difficult and potentially dangerous to cut. Leave an adequate strip of dry cloth to grip on to and do it when it's tacky and rubbery.
And there you have the reinforced seam which is hopefully completely sealed and will resist the formation of stress fractures.
I had initially planned to just fiberglass the seams and then seal the rest of the tank using Pond Shield epoxy. However, after reading some accounts of people running into some leak issues using Pond Shield I decided to first fiberglass the entire water portion of the tank using West System 105/206 and lightweight Bondo brand fiberglass cloth. I felt that this would provide structural strength and an additional layer of waterproofing. Plus, now that I'd gotten the hang of it, fiberglassing was actually quite enjoyable... almost addictive as observed by my wife
If I had more epoxy and fiberglass I'd probably glass the entire interior but I don't want to spend the extra money and I certainly don't think it's necessary.
Anyways, here's a piece of cloth trimmed and layed out
Wetted out with an initial layer of resin. I used a bondo spreader to wet out the cloth and a small brush to do the edges
After it gelled I trimmed of the excess and then applied 2 more coatings with a roller to fill the weave. Here's the tank with the lower half all glassed up. It's almost hard to tell because of how clear it gets!
After the epoxy and fully cured I spent several hours carefully sanding the tank with 60 grit sandpaper. I used a sanding sponge and wet sanded by hand to keep down the dust. This should also help to completely get rid of any amine blush, which can prevent the next layer from adhering. I carefully inspected all the surfaces to make sure there were no glossy areas. I've read the main thing that causes issues with adherence is inadequate surface preparation so I really took my time at this stage to make sure everything was well scuffed up.
Then I applied my Pond Shield. This stuff was a little tricky to work with. It's thick - kind of like honey. As per the instructions, I thinned it out by adding about 8% ethanol which made it a little easier to deal with. I calculated how much I would need to cover each side and then did one side at a time, mixing up only enough epoxy to cover that side. I rotated the tank as I went so that the side I was working on was on the bottom. I think this made it easier to work with. I followed the instructions and first used a bondo spreader to spread it out and cover the entire surface. I then used a roller to evenly cover the surface. I used a cheap polyurethane roller, which I regret now, because some little bits of the roller pulled out and got stuck in the epoxy, leaving some bumps. So lesson learned - use a high quality short nap roller.
Here's the tank with the initial coating.
After I was done there was a bunch of "fish-eyeing" and pinholes in the coating so I had to go back over and patch them with more Pond Shield. At that point the coating looked pretty good under regular room light but when shining a really bright lamp on it I could make out some areas had sagged a bit and the coating was a little thin (I could faintly make out the wood color under the bright light). This probably means I didn't quite get to the recommended 10mil thickness in those areas. I guess this happened because I was thinning it a little with alcohol but I think it would have been really hard to work with unthinned. I scuffed up the areas with 60grit sandpaper and recoated but I was running low on Pond Shield so are probably still some thin areas.
If this was over bare wood I would be a little worried but since I applied the layer of epoxy and fiberglass underneath (which I'm glad I did!) I think it should be ok. There are some thin areas over bare wood in the upper part of the tank but since they're not going to be submerged I don't think it should matter as much since all I really need is a moisture/humidity barrier, not a true watertight coating.