I've done some more indepth research on custom sumps and wet/dry filter setups and designs through various aquarium hobbyist and reef keepers' web sites. I've gained a lot of valuable knowledge while researching and then I came acrossgrifloki's YouTube channel where he demonstrates a fully customized turtle tank using the mechanics I've described.
After reviewing grifloki's video and researching various sump and overflow box designs, I was able to incorporate grifloki's setup into a draft schematic for the aquatic turtle vivarium I wish to build:
While I clean up the schematics, I am also waiting for my 12.5"x12.5" Exo-Terra terrarium stand which I've ordered from Petsmart to compliment my 6.6 gallon Fluval Chi aquarium. The set will be situated in the living room. I like the minimalistic design of the Chi tank which, along with the black terrarium stand, will compliment the living room's scheme very well.
Since the tank is so small (10"x10"x16") and the top is open (no lid), the choice of fishes that I can put in it will be extremely limited. The rule of thumb for fish keeping is 1 gallon for every 1" of fish. Which means I can keep 6-7 small fishes. I'm deciding between 6 neon tetras or 6 danios. Fancy guppies came to mind too, but I think they're way too mundane. Maybe 3 mollies or 2-3 mid-size goldfishes.