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.