Possible New Tank

I actually really like my 15's. The back is painted black and since it's not really tall, the tank looks so much deeper. I'm in the process of growing in a bunch of crypts and it looks very nice.

Iriatherina werneri- threadfin rainbows or Pseudomugilidae gertrudae- blue eye rainbows would make a nice display for the middle/top. The males have beautiful displays. Plus, they're a little out of the usual rasbora/tetra schoolers.

Pseudepiplathys annulatus- Clown killies are pretty cool and they'd hang at the top, so they wouldn't interfer with your cories, badis, crays, whichever you choose.

i like the look of my 20H...just looks more proportional to me. the extra volume might not give you any extra space for fish, but it will give you more stability.

Characidium fasciatum would work well as a group of 10 or 15 if you could find them.

Sparkling Gourami (Trichopsis pumilus), could work well as a small mixed sex group.

You could do three or 4 female Betta's, you'd just have to make sure there were lots of hiding places and floating plants right away.

Porthole and Flagtail Porthole catfish fit you size requirement, but are a bit bigger. Quite active.

A male/female pair of German Blue or German Gold rams could look nice and might spawn. Same for Bolivian Rams.

I've been breeding clown killies, they are awesome little fish. I think they might look a little out of place with cories other than the hasbrosus though, the killies are just so small.

personally, I would push you towards a species only tank. all cories, in a tank made just for cories, would be great. maybe an overhang in one back corner for hiding, some low plants and lots of sand... or for clown killies, a rocky bottom with the surface mostly covered in floating plants...

another way to go, which could be really nice in a 15 would be a river tank, with a directional current for loaches or gobies.

in a 20H, I'd do a paludarium for crabs like the geosesarma species, with maybe a few lampeye killies in the water. Lampeye killies are great, especially when they are young.

If I could restock my cork tank from the bottom up, I think it would either be a dedicated shrimp tank for filter feeders and some softer water species, or maybe I'd just get rid of everything but the microrasbora kubotai and get more of those... or maybe I'd move my lampeyes in there... but it's not really a great breeding tank, too much flow. It really requires tiny fish. I knew it would, but my SAE is already getting too big for the tank, and my Harlequin Rasboras look too big in it, in terms of the scale I was trying to create. It would look great full of clown killies, but they'd hardly make use of most of the tank, I imagine they'd be at the surface most of the time, and in the back corner the rest of the time. A 20 long would be better for clowns.

My advice in terms of building something similar, is that my tank probably could use more individualized pockets of open space, with places to hide in between. As it is, it's like a cove, with all the hiding areas along the back, and in the back left corner... It's not the biggest deal, but behaviorally, the cherry barbs are the only ones who really cover the whole tank. One thing that's tough for me, is that I can't see the filter intake cavity. Things hide there, debris gets lodged there, and dead stuff ends up there. I like that it's hidden, but it requires diligent maintenance, and careful hands. Caves and tunnels sound like a great way to use the cork, but I'd think about how you plan to maintain the areas, before you build.