Is there anything else in the tank? Is it cycled? Is your tap water hard or soft? All these things will influence what fish would be good in your tank.
PRobably a bit small for Danio then. You'd probably be ok with a group of male guppies, they're pretty active. But post up the info that Fluttermoth asked you for, and you'll get much better advice.
A tank of 30inches is really not large enough for a standard convict (or is it a short-bodied type like the ones I keep?) so you will have to either upgrade to a larger tank or rehome the convict when it has grown a bit.
unfortunately that is not a very good mix of fish. At some point those convicts will start to become agressive and your siamese fighter wont stand a chance. Do you know exactly what 'shark' you have?