I'd discontinue the water treatment until further notice. Clean pure water only.
Also please check to make sure that the brooder temperature isn't too hot. Make sure that the brooder is completely dry, no spots where water spilled into the bedding.
Yes, you can give yogurt (or another probiotic) from day one as long as you're not using any -cycline or -mycin medications. Make sure that your feed is medicated with amprolium. Anything else is an antibiotic in chick (chicken) feed. You'd want to stick with the amprolium treated only.
Use very little yogurt for younger chicks. One teaspoon per six chicks is my day 2 ration. For 9 days, you can probably do four to a teaspoon. Plain only - no bits, no flavors. Once daily.
You can also grind some oatmeal into a powder and mix a little bit with their yogurt. Or feed them a spoonful of babyfood oatmeal jarred or powdered/hydrated per four chicks at this age. It can help solidify their droppings as well as feeding the good bacteria that you're encouraging in their gut.
Good bacteria help the birds resist infection by literal competition for space. A strong colony of good bacteria in the gut will out-compete bad bacteria and fungi/yeast in normal amounts for that same space. Additionally they produce B vitamins (vital for correct development), and enzymes that help the bird literally absorb the food. They are the workers that feed your bird. Chicks are born without a colony of good bacteria in their gut. In the old days, they 'infected' themselves with GOOD bacteria by pecking their broody's fresh droppings and vent. But now instead we offer them probiotics. that's the method of how it works.
Yogurt provides D3/calcium as well for good development. But because it's a milk product, don't over do it.
If your birds continue, consider treating for coccidiosis. Coccidiosis doesn't necessarily require blood in the poop. BUT you don't want to kill the cocci completely from day one because they're required so that the chicks can develop an immunity to them which will serve them (for that species of cocci) through adulthood.
Hopefully the yogurt will help them if they'r ebattling cocci on an immunity-building basis, and the oatmeal will help with solidification of the droppings and with helping build more good bacteria.
Please feel free to email me if any of this was unclear or if you have any other questions, or just ask here.
You'll get lots of very good advice from these folks.
