Thank you for sharing the results.
I'm so sorry to hear about the Marek's diagnosis.
You still have one that is not well, what's going on with her? Sometimes treating symptoms or just offering supportive care is all you can do when one is down. Disease like Marek's can weaken the system...
I'm sorry for your loss.
It's good that you will be having further testing. If you will, please share the finding so we may also learn something as well.
If you have Fish Mox, then give a direct oral dose instead of dissolving it in water.
Dose is 57mg per pound of weight given orally twice a day for 7-10 days. If I had 500mg capsules, I'd mix it with 1tsp coconut oil, mix well, then divide it in 1/2. Roll each 1/2 into a pill form, then pop...
Did she seem to improve after you gave the antibiotic?
If so, then I'd consider treating the other hen that is declining to see if that helps.
If you have Amoxicillin in pill form, dose is 57mg per pound of weight given orally 2X a day for 7-10 days.
If you have the water soluble packets...
Welcome To BYC
How old are they?
What are you feeding, including treats?
Photos of the hens and poop may be helpful.
Have you added new birds recently?
Any access to compost, moldy or spilled feed, toxins (fertilizers, sprays, etc.)?
You put Amoxicillin in her water - dose and duration...