Hopefully your vet is open to learning, otherwise you need to find a better vet for rats.
Never put anything, especially not medication, in drinking water because it may cause rats to stop drinking and become dehydrated. Dehydration kills.
The amount of medication given depends on the weight of the rat and the strength of the medication.
If you put meds into their drinking water you may be giving too much or too little. Rats need to get a certain amount of doxy twice a day so everything that vet said is incorrect.
Rats do not get wet tail - hamsters do, so your breeder knows nothing about rats
If she does have diarrhea, we can make some suggestions re what to do.
If she is still on doxy, you will need to buy a digital scale that weighs accurately to 1 gram and has a tare function. This is an important thing for rat owners to have - they are sold in stationary stores, grocery stores, Walmart, etc
Once you know her weight we can let you know how much doxy she should be getting
How old is she? Asking because doxy is not a baby friendly antibiotic.
I hope her abscess is doing better