I'm Ed, new here, joined coz I read your plans for an African tank and just thought id throw my experience out there. I've had my fair share of African fish, so I figured why not.
First off, I gotta say, Congo tetras might not be the best choice for a 180L tank. They get big, way too active, and need to be in large groups. I'd say a minimum of 6, mixed sex, but even that's a risk. Good Congo tetras ain't cheap, so it's up to you. Maybe African red eyes would be a better fit?
Spotted bushfish, on the other hand, grow up to 6 inches and get territorial, so maybe just one would be a better idea. I've also had good luck with seSnapl bichirs, they grow up to a foot, but they're not too active, so one might be okay in your tank. A large school of true upsidedown catfish would be awesome, but don't get both, they need their space. African vampire shrimp are also a good choice, they grow up to 6 inches.
I've gotta warn you about kribs, though. When they breed, they're highly territorial and will take over your tank. They won't bother the butterfly fish, but they'll batter most other fish. I've had them in my 500L tank, and even with just a single breeding pair, they were brutal. I've been tempted by blockhead cichlids, though, they're unusual and not as fast or agile as kribs, but being cichlids, expect some aggro when breeding.
I know you don't want to hear this, but I think the usual African oddballs need a bigger tank. Knife fish, elephantnose, I've got 'em all, and personally, a 300L tank is the absolute minimum.
My suggestion would be 6-10 African tetra (if you can find something smaller than Congo tetra, I know of African yellow fins and red eyes, but I don't know about their size or activeness), another butterfly or two, 1 spotted bushfish, and 6 or more upsidedown catfish. The bushfish would be the centerpiece, they've got a great personality, comparable to cichlids, and are very friendly with their owner.
Alternatively, you could go for a seSnapl bichir or a pair of blockhead cichlids. They'll require a soft sand substrate and caves, but upsidedown cats just need plants and wood to hang around. They need to be kept in groups to be happy and active.
One thing to keep in mind is that most African fish require subdued lighting, or you might never see 'em from hiding. Floating plants do the trick, though.
African fish are amazing and unique, and I'm planning a monster-sized 850gallon tank to capture a slice of the Congo in my living room.
If you're looking for some African ideas, I'd recommend checking out these links:
http/
www.practicalfishkeeping.co.uk/content.php?sid=4153
http/
www.practicalfishkeeping.co.uk/content.php?sid=3616
I've also got a vid of my current African community tank, it's a 500L tank, but I've got a 850gallon tank planned.
http/
www.youtube.com/watch?v=COkOoOSaog0&lis ... re=mh_lolz
Hope you have as much fun as I have had with African fish. They're expensive, but so worth it.
Any questions, please ask.
P.s. Just a heads up, African fish are a bit of a handful. They love their food, and they're very sensitive to water quality. But hey, that's all part of the fun, right?