Can you post a picture of your duck's bumble?
I have successfully treated bumblefoot by just soaking their feet in espsom salt water twice a day for about 10 minutes, then cleaning the area thoroughly with chlorhexadine (any wound cleaning product safe for treating animals will do here though, we are just very limited in what is available over-the-counter in Sweden), and then I applied antibiotic gel (without pain reliever). It sometimes takes awhile, depending on how large/severe the bumble is, but it does work!
Antibiotics are a strictly controlled substance here in Sweden, so those are not available without a prescription from a vet. And even if you go to a vet, it's very difficult to get one to subscribe it for a duck. Nearly impossible, so I have had to make due without. Even the antibiotic gel I use is not available here, I have my mother ship it over to me from the US.
ANYWAY, sorry to ramble, but that is why I can't give any antibiotic advice. Hopefully someone else will chime in who can help with that for you.
A very important part of treatment is finding out what caused the bumble in the first place and fixing that problem. I realized it was the fact my old duck yard had become very hard, compact dirt with very little grass left. So, I built several additional pens with nice grass so that I can rotate my ducks around when each pen needs a break to recover from having ducks on it. Works great so that now they have very nice, soft grass in all pens. If you have any concrete or other hard/rough surfaces anywhere that your ducks hang out, you will want to cover that with something softer (grass, wood shavings, rubber mats, etc) so that they don't continue to develop bumbles in the future.
You will also want to keep the areas the duck is in super duper clean so the risk of them stepping into poo is minimal while the bumble is healing up. I got into the habit of cleaning their duck yard and house several times a day to pick up all the poops. You want their treated foot to remain clean as long as possible each day. Which I'm sure you know is a difficult task indeed for ducks.
But if you stick with the treatment routine, fix whatever caused the bumble in the first place, and keep their areas super clean while the foot is healing, you should be ok in no time. The worst case of bumble I have had took several months to treat and heal up, but it typically won't take that long. It all depends on how bad it is and how well you stick to the treatment.
