Initially they ate only what was in the sump or came through the overflow from the main tank. When they started to accumulate I started adding food to the sump. Mostly pellets and flake foods. I'm not sure that they were very effective at eating any of the chaeto. They did spend most of their time pecking at it but I think they were eating new growth and/or micro algae off the chaeto. Without the mollies the cheato is a lot more dirty. So the no3 went from under 5ppm to 25ppm or so. The po4 went up from .03 to .1 over there course of a few months. It wasn't until I accumulated a lot of them that there nutrients started to raise... And I started getting a lot more algae in the main tank. When I took them out the numbers went back to low levels in a week. Some babies have made their way down into the sump again so I would guess the raising nutrients will start again eventually. I have mollies in the main tank, the babies end up eaten or in the sump. I added a sock to catch the babies but I guess it overflows sometimes. In the main tank almost everything will eat the babies (corals, Cardinal fish, copperband butterfly, 6-line wrasse, blennies, and the parents).
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