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If she were mine I would be treating her for Coccidiosis, and if you have others, treat them too. Amprollium (Corid) is the common treatment for Cocci, and is readily available at feed stores. Have you ever dewormed your flock? You can always have a fecal float done, most vets will do this for you for a small fee even if they are not an avian vet. Then you will know for sure.
You can have a float test done at the vet. Costed $25 for me.
When my flock caught cocci, I saw bloody poos in the am and regular poos witnessed throughout the day from each chick. I treated with Corid, but had the test done since the poos weren't consistent. It confirmed cocci. I started treatment before other symptoms appeared.... Standing puffed up, sleeping. Listlessness. Do you have other symptoms?
You say your chicks didn't have that before getting grit, but I see the poo is on grass. Did you move them out at the same time? If your chicks are on pasture, they probably get all the grit they need without supplementing. My chicks caught cocci about 6 days after heading to pasture, figure that might be where they picked it up since that's the incubation period.
All of my chicks recovered. I say don't wait!
When you say "away" from my other two, how far?
I didn't lose any chicks. I cleaned obsessively and followed standard treatment.
I don't think your chicks need grit, unless... you are feeding them treats AND they don't have any access to dirt. If they have access to dirt, they should get what they need. You don't have to get rid of it, I just wouldn't buy more. Starter feed has the grit in it to cover what they need if that's all they are eating.
I am no pro though, so please review any suggestions I make and know that even well meaning people mess up and everyone may have differing views depending on their previous experiences. I can only speak for me. I would probably treat the others for cocci. I had a separate set of older pullets I had to start treating a week later. There is a preventative dose.
For your younger chicks, are they on medicated or un-medicated feed? Are you feeding the layers a different ration than chick?
Hope they make it. Good luck.