What is the cheapest water soluble antibiotic?

[darlingdarla]

In mid February my two year old dark cornish hen started gaping when I handled her. She was still eating and drinking and otherwise appeared healthy (other than minor leg mite infestation which I took care of easily). I started trying to ease her symptoms with VetRx, which did nothing. She hasn't gotten much worse, today she was breathing with her beak open without me handling her but honestly that could be because of the warmer weather.
I've noticed a few other chickens doing the same thing but most of them not consistently. The only birds that gape every time I pick them up are my brahma bantam, Speckled Sussex and dark cornish, also a polish hen who just developed the gaping today.
I don't like to rush to use antibiotics, both to avoid overusing them and because they can be expensive.
I would really like a water soluble antibiotic, I have thirteen chickens and really don't want to dose each bird individually since most of them are not tame (though I can if I need too). I also need a low cost antibiotic, I'm not working due to covid and need to save every penny (though if saving money is decreasing the effectiveness of the treatment I'll buy whatever, just figured I'd check here first).
Thanks in advance, and sorry if this post is scatterbrained I'm having trouble collecting my thoughts.

 

[Wyorp Rock]

The only birds that gape every time I pick them up are my brahma bantam, Speckled Sussex and dark cornish, also a polish hen who just developed the gaping today.

started gaping when I handled her

How old are these birds?
Do they lay eggs?
Any bloat/swelling or fluid in the abdomen?
Crops emptying overnight?

Hard to know which antibiotic to even suggest without knowing the cause of the gaping.
If they are not gaping all the time and only when held...then I would not treat. Birds that have respiratory disease will have symptoms all day/all night.

 

[darlingdarla]

How old are these birds?
Do they lay eggs?
Any bloat/swelling or fluid in the abdomen?
Crops emptying overnight?

Hard to know which antibiotic to even suggest without knowing the cause of the gaping.
If they are not gaping all the time and only when held...then I would not treat. Birds that have respiratory disease will have symptoms all day/all night.

The birds are five, two and one year old.
They all lay eggs (except for one five year old who has a reproductive disorder).
No swelling in the abdomen.
Crops all seem good.

There's also no growths in their mouths as far as I can see and I have swabbed their throat for gapeworms (though I don't believe their symptoms to be consistent with gapeworm). I've been keeping a close eye on them, I have so much time right now I've spent whole days watching them and in the end I just have a bad feeling.

 

[Wyorp Rock]

The birds are five, two and one year old.
They all lay eggs (except for one five year old who has a reproductive disorder).
No swelling in the abdomen.
Crops all seem good.

There's also no growths in their mouths as far as I can see and I have swabbed their throat for gapeworms (though I don't believe their symptoms to be consistent with gapeworm). I've been keeping a close eye on them, I have so much time right now I've spent whole days watching them and in the end I just have a bad feeling.

Can you get a video of them gaping?
Do they gape all the time or just when handled?

 

[Eggcessive]

Are your hens overweight? What breeds are they? Is there any wet or moldy bedding or mold in your coop? Coop ventilation should be open overhead with end to end to end ventilation. Is it hot there? Do you see any sneeze, head shaking, or watery eyes or nostrils?

 

[darlingdarla]

Can you get a video of them gaping?
Do they gape all the time or just when handled?

I can try to get a video, I tried to film them this morning but as I said my flock is not tame so I got some nasty scratches for my trouble.
It's only when I handle them, yesterday one started gaping when she's just out walking but it's very subtle.

Are your hens overweight? What breeds are they? Is there any wet or moldy bedding or mold in your coop? Coop ventilation should be open overhead with end to end to end ventilation. Is it hot there? Do you see any sneeze, head shaking, or watery eyes or nostrils?

They are not overweight as far as I know though I haven't weighed them in a few months.
The three that have consistent gaping are my two year old dark cornish, my one year old buff brahma bantam, and my one year old Speckled Sussex. The girls I have noticed it in but inconsistently are my black copper marans, my blue copper marans, my buff crested polish, my porcelain d'Uccle bantam, and one of my easter eggers.
I don't think the coop is wet or moldy I clean it as frequently as I can, though it has been a little longer than usual because of the quarantine I bought bedding to clean it this weekend.
I don't think my actual coop is moldy, but I'll look underneath it this afternoon. Also their ventilation is overhead and it is clear.
It hasn't gotten hot here yet, we've had a few warmer days but this started in February when it was still cold.
They sneeze occasionally but nothing that's ever worried me, head shaking I haven't noticed and no watery eyes or nostrils.

 

[Wyorp Rock]

It's only when I handle them, yesterday one started gaping when she's just out walking but it's very subtle.

my flock is not tame so I got some nasty scratches for my trouble

The video would be good.
Birds that are not tame and are captured often do gape due to stress.

I'm just trying to figure out if the gaping is from stress of handling or if they have health issues. If they are heavy/have fatty abdomens, maybe a little fluid in the abdomen, etc. Depending on how they are held they may also gasp a bit. Crops that are full, then pressed when picked up/handled, birds could aspirate a tiny bit of fluid/water and gape.
Most of my hens handle well, but I've had some that are flighty. If I need to treat those for some reason, I wait until they are roosting then take them off the roost - either at night or go out really early. They are usually calmer then and I don't have to chase/grab them which is stressful for them and not fun for me either LOL

 

[darlingdarla]

The video would be good.
Birds that are not tame and are captured often do gape due to stress.

I'm just trying to figure out if the gaping is from stress of handling or if they have health issues. If they are heavy/have fatty abdomens, maybe a little fluid in the abdomen, etc. Depending on how they are held they may also gasp a bit. Crops that are full, then pressed when picked up/handled, birds could aspirate a tiny bit of fluid/water and gape.
Most of my hens handle well, but I've had some that are flighty. If I need to treat those for some reason, I wait until they are roosting then take them off the roost - either at night or go out really early. They are usually calmer then and I don't have to chase/grab them which is stressful for them and not fun for me either LOL

I guess it could be stress, it's just never happened before and I've had them for years now. Nobody has any swelling or are overweight. So I don't know maybe it's all in my head, I feel like I'm spending my whole day just staring at them I guess anything can seem like a health problem if you stare at it long enough.
One thing I noticed today on my black copper marans, a lot of the feathers around and below her vent are missing, her skin is red and there's a whitish crusty discharge.
As soon as I noticed it I went looking for mold or fungas or something and at the bottom of my trashcan where I keep the feed there was a bit of growth. It must have leaked during one of our recent storms. It came through the bottom of the feed bag. I threw out what feed was left, I've spent all morning raking out the run and cleaning waterers. I sent my brother out to get fresh feed and I'll be cleaning the coop tomorrow morning. I checked the rest of the flock as well, one other hen is missing some feathers but not nearly as many as my marans and three other hens have messy bums but no feather loss or redness. None of these hens are gaping.
I got some pictures, my first thought is vent gleet, but I've never dealt with it before so I can't be sure. I didn't find any external parasites and I doubt it's feather picking as this girl is near the top of the pecking order.

 

[Wyorp Rock]

Looks like a messy bottom and crusted poop buildup.
She may have a little extra fat or fluid?