My delhezi is almost constantly hunting my congos for the last several months and I've only had one get eaten.
Make sure you have a good number of congos, I think they kind of startle as a group as soon as one notices the poly ready to attack. I also noticed they'll hang out near the bottom if they're not in a group, which is prime bichir attack territory. In 30 gallons you might just not have enough room though.
I keep mine in 75, soon to be upgraded to a 120 or 150.