one of our 6 day old chicks just started having bloody poop...
she is also lethargic…
Sounds like its Coccidiosis you need to immediately start treatment with Corid. I would make the solution available for all your chicks.
Corid can be found at Tractor Supply or your local feed store.
Here's the dosing information which is referenced from the below links.
1 1/2 teaspoons (NO LESS) Corid powder per gallon
2 teaspoons of 9.6% Corid liquid per gallon
Give for 5-7 days... make sure this is the ONLY water available during that time period. Mix a fresh batch at least once a day.
Also clean out the feeders and waterers and disinfect (I recommend a hydrogen peroxide solution). Clean out the brooder tossing the litter in garbage bags and not your compost pile.
Coccidia are protozoa that live in the soil and feces. They multiply exponentially, so once you've got an overgrowth going, it will quickly outstrip the environment.
All soil has coccidia, and all birds have a little coccidia in their systems, but they develop an immunity to it which means their immune system and healthy gut flora keeps it in check. This can become imbalanced under stress, warm/moist/dirty conditions, and any bird that has changed locations and not had time to build up immunity to the new strain in the new soil/environment.
If you have other birds, be very careful to use good sanitation as you can track coccidia to other areas and start an overgrowth there.
Typically the brooder is notorious for "brewing" all manner of microbes as it is warm, moist, and filled with pooey little creatures who are new to the environment.
EDITED TO ADD: after your course of Corrid, you may want to place all chicks on medicated (Amprolium) based feed. Medicated chick feed is low dose Amprolium that prevents overgrowth while the chicks slowly develop immunity. If you've been using medicated and still had a problem, then even more reason to aggressively and diligently clean your area.
I wouldn't recommend ACV use with Corid, or while they may have an intestinal infection such as coccidiosis due to irritation of the gut. There isn't any evidence to show that it prevents cocci, but a lot of people use it. Probiotics in the diet can help, and if your ACV has the probiotics from the mother it might help. Probios powder and some chick vitamins, along with many of the brand name feeds (Purina, Nutrina, etc) now contain probiotics.
I personally have done it both ways, as sometimes I don't have an extra waterer for ACV as I do not put that in the same water as the Sulmet or Corid (I prefer Sulmet to Corid if I've got a nasty coccidiosis outbreak...but use what you've got)....but I believe it really is best to continue ACV even during a Coccidiosis outbreak...it is not a matter of inflicting more irritation to the gut but helping in gut repair itself (see articles below for the reasons behind that statement).
So if I don't continue ACV during the med portion, I make a point to put it back into their regimen quickly after the med rounds.
Will ACV prevent or help cure coccidiosis? The simple answer is no, however a healthy gut is the first step in a strong immune system, and good gut flora is essential to gut regrowth.
Coccidia do not, to my understanding, compete against other bacteria or yeast, so building good bacteria won't prevent coccidia by competition, but a strong gut flora will help keep all the "bad" in check and encourage rapid gut repair....something likely important during the devastation the coccidia are doing to the gut wall while you bring them back under control. (Regular amprolium based feed is important during the growth periods for most birds to keep coccidia in check).
ACV (an acidifier with most importantly the beneficial "mother") not only helps to increase the acid in the gut (important for gut health), but the mother also provides nutrients that good bacteria feed on ...so the ACV is helping to acidify the gut to keep bad bacteria in check, feed the good bacteria, all in turn improving the flora so that it in turn helps in rapid gut repair and immune system response.
Make sure the ACV is of good quality...raw, with mother, and never fed in a metal can as the zinc coating will leach into the water causing toxins. I heartily agree that providing pre-biotics and pro-biotics is also essential as the birds rebuild their guts during the effects of this illness.