You need a prescription to buy antibiotics, unless you buy them online but if you buy online they might not be what you think you are getting, they could be mislabelled, a totally different medication, fake or expired. Some people use fish antibiotics but they are not safe to give to rats as they are not strong enough to treat rats and can cause the rats to become even more ill.
Plus when rats are ill, they really need to see a good vet so that you will know exactly what is wrong and get the right meds for them. The amount of meds they are given depends on the strength of the meds and their body weight so having a reliable digital scale with a tare function is a good thing to have.
So, you need to find a good vet that is very knowledgeable about rats or is a good vet who is willing to learn. Having a good vet is essential because rats are prey animals and hide illness so when you notice they are ill they are already very sick and die fast. If you establish a good relationship with your vet, you may be able to get meds from your vet to have on hand.
Rats are not inexpensive. - Medical care can be expensive and ill rats need immediate treatment so it is important to have something in place to pay medical bills. It is strongly suggested that rat owners have a bank account with at least $300 for the first rat and at least $200 for each additional rat .... and that the money is replaced as it is spent. Some people such as myself have a credit card to play for medical expenses.
From what you wrote they are likely ok, but if they are ill or do become ill, there are antibiotics that are safe for young babies. If you recently got them, they could be stressed by the change.
The red discharge is likely porphyrin as you thought - that means they are stressed and may be ill so keep a close eye on them. Spending a couple of hours daily playing with pet rats is important for many reasons and it also lets you know if there are any changes in their behaviour so you will know when something is different so you can tell when they are ill.
I am confused, how can they be 6 weeks old if you have had them for a month? Did you rescue and hand feed tiny babies? Babies should never be rehomed until at least 6 weeks old and baby boys stay with mom and sisters until 5 weeks old ...... although people who don't care or don't know much about rats do take babies away from their mom too early.
There is a lot of good info and ideas on this forum. Here is a good site for info
http://ratguide.com/
hope this helps