Occasional brown smelly poop is normal- this is cecal poop. If all of the poop is like this, or one of the chicks has only this- that is abnormal. Diarrhea can be caused by many things (just like in people). Bacterial infection, parasitic infection, viral, food ingredient. Young birds will also get diarrhea if too hot or too cold! Describing your brooder would be helpful- temps especially. 7 week olds should be able to take 60-65 if there are a bunch to keep each other warm, but having a warm spot for them is nice. Are they bunched/piled and peeping, or spread out when they sleep? If the whole brooder is hot- they can get diarrhea. If you got a new food right when the change happened- you can try switching back. If you want to know if there is a parasitic infection for sure and not just guess- you can have a fecal sample checked at a vet's- but in a 7 wk old bird, worms are unlikely. Coccidia (a protozoal infection) is common in chicks in some areas (not sure about Hawaii). This would also show up in a fecal exam- it usually causes diarrhea with blood flecks. OTC treatments for this are sulfa drugs (sulfadimethoxine) and amprolium. Dirty water can cause bacterial GI problems, can they poop in the water?