Originally Posted by eaturbyfill
I read that Zebra Danios are often aggressive-- is this true? I do not want any conflict between the Danios and Rasbora. Also, one site said that Barbs should not be mixed with other fish because of aggressiveness, and another said that Barbs would be fine with others?? I don't know which to believe. I planned on keeping a school of Tiger Barbs alone in a 10g, and then the Danios and Rasbora together with some Corys in a larger tank-- now I am confused!
I didn't comment on the compatibility issues previously because you seemed decided on what you wanted, and I seem to endlessly be telling members not to combined this and that fish.
But as you have now asked...
"Aggressive" is a term that can have various implied meanings; we got into this in another thread last week. Sometimes it means physical conflict, but it can also refer to chasing, sparring, fin nipping, etc. Aggressive can certainly be applied to most of the barb species, and somewhat to many danio species. But this does not equate to "aggressive" in the sense of a convict cichlid or similar.
All barbs are "feisty" to some extent. Some are worse than others. And they are active swimmers, meaning they will be swimming actively around the tank. Danio share this latter trait (active) but are generally more "peaceful." However, peaceful is a relative term, and while we can generalize like this, individual fish can be the exception either way.
Rasbora by contrast are much less active, and almost always peaceful. They thus make better companions to sedate fish like gourami, angelfish, etc. Barb and danio should never be mixed with sedate fish. The level of activity would likely bother the sedate fish which causes stress (thus poor health), and there is always the tendancy to fin nip.
The "feisty" fish like barb and danio need more room, because of their more active lifestyle, and also because their feistiness is less damaging in larger surroundings.
Now to your specific questions. Yes, I would not myself mix danio in with rasbora in small spaces--and a 30g is small to the fish. In a 4-foot tank there is better chance of this working, but still I prefer to keep "sedate" and "active" fish separated. I'll digress for a moment.
I have a 90g tank which I call my Stream habitat. I specifically aquascaped it for "active" fish, and it contains groups of Congo Tetra, Black Ruby Barb, Emperor Tetra, Golden Pencilfish, Zebra Loach and Angelicus Loach. All of these are active swimmers, and this tank is a true hive of activity. But the fish are all peaceful. They have their social structure within the species, esp the loaches, but this is normal and no physical damage occurs. I did for a time have my Hengels Rasbora in this tank, as i was setting up a 30g into which i moved the rasbora. Clearly they were not happy; they remained together, generally close to one corner. Nothing attacked them, but the constant activity around them obviously bothered them. They are now in the 30g and much more relaxed.
By contrast to the active tank, I have a 5-foot Amazon Riverscape tank, with "sedate" fish by comparison. False Rummynose Tetra, Brilliant rummynose Tetra, Roberts Tetra, Bleeding Heart Tetra, one Bolivian Ram, Marble Hatchetfish, and over 30 corys. Even though the rummys like to swim, and they do, they are not anywhere as active as the Congo, barb, etc. in the other tank. This Amazon Riverscape is much more tranquil. The shoals of fish just cruise around, gather under plant leaves, etc.
The success of a mixed community depends upon several factors, and activity levels is one important one.
Back to your situation. The Tiger Barb need no less than a 30g, and a group of 8 or more, on their own. This is the minimum. In larger tanks, they can be combined with other species. This is set out in the profile (click shaded names for profiles).
Hope this is of help to you.