I've seen production tanks with a bubble or two in an inner seam, but that one got by the QC person completely. There was never any silicone applied between the two panels. Somebody forgot and decided to just apply an outer bead and move on to the next one. That tank was relying on that one-piece rim more than you think.
If you can run wire between them that's a huge gap. You shouldn't be able to fit the point of a razor blade in there.
Honestly, if it were mine, I'd put the rim back on and take it back to the store and see if they'll give ya credit or sumthin. New tank prices can be all over the place, but I wouldn't put money into making it structurally sound. If they won't take it back set it up somewhere where a leak won't do harm. A 20G will never develop enough pressure to just blow a panel. Petco's $1/gallon sale's coming around soon.
In short, I wouldn't trust it at all without a rim even if you squish silicone in there. You'll never be able to clean that gap completely without removng the panel and scraping the silicone and reapplying it. You could tape off the joint so it comes out neat, but the bond would never be completely trustworthy in my opinion and it's just way too much work invested in a 20G.