At six weeks old, I'm going to guess that you've allowed them out into the yard. Has it rained recently? Every bit of soil on the planet has coccidia, and worms are easily carried by wild birds and pooped into your back yard... so if they've been exposed to grass, dirt, or the great outdoors within the past couple of weeks, you can expect them to be a bit 'under the weather'.
You may be dealing with worms (which usually are invisible to the human eye) or coccidiosis.
Both problems are common with young birds or with adult birds that are either old or have underlying health concerns.
Fortunately, you can pick up medicine at your feed store for both, add that to all of their available water (at the same time). Do NOT give vitamin supplements while administering amprolium as it works by mimicing the B-vitamins that coccidia consume as food. No B-Vitamins, and the parasite starves to death. Do offer vitamin supplementation after the full course of treatment. Ask your feed store guy for poultry worming medicine, for Corid (amprolium is the generic name), and for the vitamin supplement that will be given after their treatment is complete. These three things ought to set things right with your young flock.
Keeping you and your feathered family in my prayers.
