You didn't say which species of danio, but all of them are shoaling fish, meaning they need a group. While six is usually considered minimum number, more is better.
Shoaling fish occur in large numbers in their habitat. This affords protection, but also they frequently (depending upon species) have a social structure within the groups. A recent (and the first) scientific study has proven that when shoaling fish are kept in smaller groups, less than 5-6, they frequently become more aggressive. Smaller tanks also cause this. Part of the reason is stress; not having the fish they expect around them and having too little space is stressful, and this leads to aggression (even in otherwise peaceful species) and a weakened immune system. I suspect this is a circle in effect.
If you check the species in our profiles section [second tab from the left in the blue bar at the top] there is info on minimum tank sizes [for only the fish species, additional fish in the tank may require more space], minimum numbers where applicable, and suitable companions. Also water parameters including temperature are given, and as someone mentioned earlier, fish kep at inappropriate temperatures can also be stressed and...here we go again.
And corys are shoaling fish, and should never be less than 3 in an aquarium, preferably 5.