You could use a small aquarium sponge filter/air pump combination. Rinse the foam filter in aquarium water (i.e. during a water change), not in tap water that will kill the beneficial ammonia/nitrite reducing bacteria. What is their current set-up and how/when do you conduct water changes?

Can add veggie type fish flakes to their diet. Tadpoles consume mainly detritus, algae and higher plants. Also, animal matter can be consumed in smaller amounts. Don't think they became predatory to the shrimps. Maybe the shrimp died and then the tadpoles consumed their organic matter. Good luck !