11-24-2011, 01:17 PM
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Welcome to Tropical Fish Keeping forum. I have moved your post to its own thread.
To your question. There are some issues with the mentioned fish in the same aquarium. Tiger Barb should be in larger groups to reduce aggression, and on their own unless the tank is quite large. You can read more in the fish profile. Profiles are under the second tab from the left in the blue bar across the top of the page, and in each profile there is data on water parameters, tank sizes, numbers, compatibility, etc. If the name is used exactly the same in posts as in the profile it will shade and you can click on that for the profile.
By "red robin" I am assuming a red form of the Dwarf Gourami. This is not a particularly good community fish, again there is more in the profile.
"Plecostomus" can refer to many different fish, some such as the Bristlenose Pleco remain smallish (around 4 inches) but many like the Common Pleco grow well over a foot in length. A small species would work in a 30g.
Corydoras are shoaling fish and are highly social so a group is better. In a 30g you could have 5-6, either 5 of the same species of two species with 3 each. I would not recommend less than 3 of a species, though 2 is OK if there are other corys.