Honestly, I was a bit scared of epoxy. Fast setting time, almost explosive heat development, and brain cell damaging smell was what I had read. None of that was quite the case. After a while I figured out that it is just some glorified glue. I spent some time to research which epoxy would be the best for my situation, and ended up with Coat-It made by Goop.
Compared to the more obvious choice (West Systems) this epoxy comes in at half the cost, has a dark coloration, and requires only 1 to 2 coats. It has a very syrupy consistency, wetting out fiberglass mats would be almost impossible. It also makes painting vertical surfaces quite easy. The smell is very benign, I suffered no immediate brain damages besides a very light headache in my not so aerated garage. After 12 hours Coat-It starts to get less tacky, and after 2 or 3 days it feels dry.
Important things with epoxy - take the right proportions, and mix well. I got an 8lb kit of Coat-It which covers around 80sqft (about the size of my project). 7lb resin and 1lb hardener, so it needs to be mixed 7:1, which was simple enough with a little scale that otherwise weights fertilizer powders. One of the things to avoid with epoxy is to get it on your skin... can you say itch & burn? I wore two pairs of gloves, thin latex underneath some heavy duty ones. The latex ones help to get the thicker ones off without touching any epoxy.
The dark coloration makes it easier to see where the coat is too thin. I did two layers in most places. To apply it, a fairly stiff bristled brush that didn't shed hairs worked best for me. Neither freezing the brushes nor keeping them submerged in paint thinner kept the epoxy from hardening, so I just threw them out after each painting session.
Like I mentioned earlier, kept both tanks and the back separate to make painting easier. With the filter compartments, it would have been difficult to paint with the back panel attached.
After silicone-ing the front glass panels, I will screw/glue the back to the rest of the construction and paint just where the corners meet. Already drilled a hole into the garage wall for the overflow. Time to think about how to lift the whole thing up onto its concrete base. I'd like to suspend it in the air somehow, while adding some thinset, and then lowering it down into the fresh mortar to provide the most level surface.