Smallest Predatory Fish?

[ZitoDragon]

Hiya!
 
I was going to turn my old thirty gallon (currently holding a lone pictus cat. It might actually be larger than thirty, its not at my dorm right now and I only eyeballed it.) into a saltwater aquarium for a mantis shrimp. I have had reptiles in the past and seeing them hunt insects in a simulated natural environment is absolutely incredible.  Unfortunately, saltwater is... expensive. So, what predatory fish could I stock my tank with?
 
Currently my idea is to get another pictus to keep mine company, and a butterfly fish for the top, then one or two fish for the mid level (I have no idea what species.). However, if possible I would love a group of swarm hunters.
 
I have heard bucktooth tetra would be awesome (obviously the pictus would have to go) but they seem awfully big in swarms for a tank that size. Are leaf fish any fun to keep?
 
I want something big or ferocious enough to eat small feeder fish. While I'm on the topic, what would you recommend? Anything I could easily reproduce in a small (5 gallon desktop) tank?
 
Also, I'm not at all opposed to amphibians
 
Thanks for any help!
 
 

 

[BettaBettas]

what are the tanks water parameters?

 

[Munroco]

You could always just buy feeder fish and smash then with a hammer, would save time and you wouldn't have to buy any more fish to rip them to pieces.

 

[Ninjouzata]

Feeder fish are generally riddled with illnesses that could pass on to your fish so I would not recommend getting a fish solely for that. I'm a little confused by your wording, is this supposed to be for a 30g or a 5g tank, or both?

 

[fluttermoth]

Apart from the moral and humane issues, feeding predatory fish on a diet of feeder fish is not acceptable.

Most commonly used feeder fish contain a substance called thiaminase. This, over time, causes a lot of health issues for the fish being fed.

Domestically bred fish and their diet are a world away, nutritionally, from wild fish and their diet.