Duckling falling down

On and off I have been having where a duck will freeze tail down wings out try to hold themselves up and then lay down. Lately, this has been happening a lot and now in three ducks. There run is clean no mold, no chemicals are used, and their food is not moldy or old. They are provided additional brewers yeast, probiotics, sea kelp, herbs and a handful of scratch each night. I have been to the vets office multiple times and they have no clue but now I have three ducks doing it. Any help would be great they have been started on a vitamin b complex also. There is a video posted below

Poisoning in ducks
Botulism is found in stagnant water. Since ducks love to splash around, if their water is not clean they could be exposed to something toxic. Mold or other toxic exposure can lead to symptoms of a sick duck such as lethargy, diarrhea, decreased appetite, loss of weight, seizures, dehydration, vomiting, drooping wings, unsteadiness on their feet, or difficulty walking.

curious: what kind of herbs? or maybe there getting drunk from the yeast or herbs?

It looks to me like genetics. If you look at its feet, they are inward turned slightly and the duck stops when one foot steps on the other preventing it from making another step. All they have to do is make (2) left foot steps to prevent the foot jam (not to be confused with toe-jam), but ducks do not waddle like that. It is left right left right, so when they get tripped up, they stop and put their wings out in a defensive posture.

If you watch the video carefully, you will see the duck waddles, then stops when the foot crosses the other. It almost puts out its wings, but then at the last microsecond, the foot turns and it waddles ahead again. It does this a second time, starts to set out its wings, but then lurches ahead for a waddle or two, but the third time the ducks feet cross it has partially got its wings out, and just stops and gives up. Imagine if in the span of 35 seconds you "tripped" (3) times? It would get very frustrating.

I had a duck of mine do this. Eventually he learned to double-step on his own.

This may or may not be the cause. It is hard to make a determination from a 35 second video so don't tar and feather me if I am wrong (get it, tar and feather)?

Have chickens not ducks....but that doesn't look good to me. If they just started doing this (vs. being born with it) I don't think it could be a genetic foot problem.

Never heard of botulism in duck water but botulism is darn scary stuff. The toxin causes paralysis, in babies a mild dose is known as "floppy baby syndrome" since they get all floppy,  and in high doses it paralyzes the body AND the lungs and the victim suffocates (unless put on a ventilator).

My next thought was Marek's but I don't think ducks are as susceptible (it causes paralysis and weird movement issues) and that doesn't look like the type of Marek's I have seen in my birds. That starts with a limp and gets more pronounced until they can't stand at all, and it is usually triggered by high stress or a very stressful event such as integrating with a flock and getting severely picked on, or an animal attacking the flock etc.... Has there been a high stress event?

To the op I would definitely be all over this until I had a definitive answer as it seems like something that could get worse in a hurry. If this just started and is getting worse then toxins may be an issue, but I wouldn't expect a dietary deficiency to be the cause as that would likely come on a whole lot slower.

The hard thing about ducks (versus chickens) is that due to the nasty water they digest, they are immune to antibiotics so that does not help. :-(