Fish that will eat molly fry.

I inherited a black ghost knife in a 29-gallon. He'd been in there for a couple of years and stayed another year before I rehomed him with someone who had a bigger tank. He shared the tank with my guppies, and I never had a fry problem until he went to his new home. A black ghost knife won't be cheap, but you can probably get one for $25 - $30. I fed mine mainly flakes, but also frozen blood worms. Plus, he had lots of guppy fry for snacks. Hardly ever saw him, though, except when I fed in the mornings. Otherwise, he'd hide out in his log. But a black ghost knife is a beautiful fish to watch. Also, they will learn to eat from your hand. Finley was the best!

A black ghost knife will get anywhere from 16-20 inches, I wouldn't recommend this fish for a 30 gallon. OP made no mention of upgrading tanks so in the long run I think this fish would be a bad idea for him.
 
If you add a heater, how about Buenos Aires tetras? Very active, don't get bigger than 3 inches, easy care. And cheap. Full price is $2 something here, but I got mine during Petsmart's tetra sale for $1 each. Mine are still only 1 to 1 1/2 inches (I asked for the smallest ones), but since I added them to the guppy tank a week and a half ago, there are NO more fry. They get more aggressive as adults, but I've read they're okay with adult mollies. With a heater, other tetras or barbs would probably work, too, at least if the adult mollies are in a different tank. Danios or giant danios, too.
 
Yes, Convicts will breed but they are also ok alone. Something like a Paradise fish would probably eat baby mollies and wouldn't need a heater. A good sized gourami would work if you can heat it. My tiny Sparkling gourami ate up a bunch of guppy fry.
 
Blue Gularis Killies
 
Hmm, well, i could afford a heater, but new twist, its actually a 20gal tank, not a 30gal. I bet that takes out most of those fish, which sucks. As for convicts, how do you tell which is male/female? And will paradise fish breed in my tank? As for tetras, i could do that, if i go with the heater.
 
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