smaller fish that thrive well in bigger tanks...

[plebian]

It's amazing to me that questions about tank size continue to come up. BIGGER IS BETTER.

It doesn't really matter the size of the fish. There are very few species that are naturally confined to small spaces. Even Betta splendens will guard roughly one cubic meter of water in the wild. Yet people insist on keeping them in 5 gallon (or smaller) tanks. Will they survive? Absolutely. Just as a human being can survive in an 8' by 10' cell.

The smaller the fish, the larger the group that should be kept. The larger the group, the larger the tank. Spare me the rare exceptions to this almost universal rule.

 

[GaryE]

That's a small tank not a large tank.

Ah, that's a hard debate to be black and white on. A 3 foot tank is a large object. In the reality of people's homes and budgets, it's a mid-sized tank, and as large as many people will be able to get and keep. You may not consider it large enough for a lot of fish, and you have a good argument there. But not everyone is in your situation.
A 3 foot, 1 meter tank is fine for silvertip tetras. A dozen or more would be fine in a space 15 times their adult body length. A 5 gallon for a Betta? I wouldn't do that myself, but I wouldn't keep a long finned Betta bred for spaces like that. If you've kept wild caught Betta splendens, you'll know it's a very different beast compared to the overfinned, handicapped fancy forms, which often have trouble swimming properly. That's a whole different debate.

The exceptions you want to be spared from destroy your rule, which only applies to shoaling fish.

 

[anewbie]

So should i get rid of everything in my 600 and put in a single betta. Dimensions are 10 ft long and 4 ft wide.