Community tank suggestions?

Yes, cories are corydora catfish, but no, they aren't the huge things you're thinking of and they aren't messy. They are called "bottom feeders" as they are a good cleanup crew for any fish food that ends up on the bottom. Though they should be supplemented with sinking pellets.
Anyway, they only get about 2-3 inches, so you have nothing to worry about for your tank size. I love my peppered cories (4) to bits. They're in with my betta and I have no trouble. They are very cute to watch, though they should be in groups, not solo. I've heard at least three, but in your tank I'd get more. There are lots to chose from in the way of colors, but I'd get at least a few of each kind you pick.
If you go to wesellguns post above and click on his link to liveaquaria, you can then click on 'fish' at the top. Then click on freshwater, then chose 'catfish'. You'll get a better idea of what they look like, and what they need. I think you'd be happy with them.
 
Mixing Corys

Is there any problem with mixing different types of corys? I was hoping to have 3-4, but thought it might be nice to have a little variety - perhaps a panda, a skunk, an albino, etc.
 
rainbowfish!

Check out rainbowfish if you haven't learned about them yet. They are beautiful, peaceful schooling fish from australia/new guinea.

I think turquoise rainbows and neon dwarf (praecox) are the prettiest.

With a tank as large as 75 gallons I would get larger fish like this, then fill in some spaces with smaller stuff like cories.
 
*Cherry barbs not a good idea for your betta, they are prone to fin nipping as are a lot of barbs, like tiger barbs and rosey barbs etc.
as for other fish with your betta I have had them a few times, I had mine with guppies, mollies, cories, ancestrious plecs (fantastic look like little little puppies with wagging tails and they help munching on algae) dwarf gouramies. You will need to be carefull of your choices of larger fish with your betta cause he is a bit slow and prone to nipping
 
Whoa ultimate 75g tank dreams galore here. The betta should be fine if you keep the main fish to tetras. Really nice looking fish in that family as a whole there and very compatable with bettas. With a betta anything colorful and long finned is a BAD idea. Bettas can and most likely WILL see it as another male betta and kill or attack it on sight. I had a male betta in my 29 for months with neons, white clouds and corys without many problems. The problem I did have was that the corys were about the same size as him and he flared and attacked them on sight with out pause. In a tank as large as yours that may be less of a problem since corys are litterally 10x's faster. You'll be limited though in other types of fish because of the betta. My suggestion would be to get a small 5 gallon tank for him and then you can litterally open up the tank to almost any kind of fish. If you keep him in there I suggest looking at White Cloud Mountain Minows as a possibility. Really nice schooling fish very hardy and although they lack the tropical colors they are one of my favorites. Afew things to keep an eye out for with the betta though. As a breed they tend to be raised in VERY limiting spaces (i.e. the cups) so once they hit large tanks they can stress easily. They can get nervous and most likely will come down with desease. So watch the little guy for Ich and such he's most likely not going to adjust right away to his new more suitable tank size. Once he does though I think you'll be happy with his range :D .
-Neo Sithlord
 
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