You can also try wiggling the frozen food (properly thawed, of course) with a tweezer or a loop of fishline to encourage interest, as well as soaking it in garlic juice (fish love it) to induce feeding along with the above suggestion of not feeding for several days. It doesn't always work, but the combo usually gets them.
IMHO, I would breed either convicts or kribs along with a livebearer like platies or mollies. That gives you the best of both worlds, the larger more plentiful egg layers as well as the very prolific livebearers. You are almost sure to never run out of food, and it give some variety. You could do a pair of kribs with 1 male molly or platy to at least 3 (you'll probably need more like 7+) females. I'm not sure if convicts can be housed with platies/mollies or not as I have never kept them, but I do know the kribs will be fine with the mollies as long as there are caves (plain terra cotta pots work just fine).
