This sounds like fin rot rather than a true ulcer. The most likely cause is poor water condition. you really must check the ammonia & nitrite & nitrate levels if you have 2 adults & 7 4-5 month olds in a 25 gal. <BTW - how big are the fish?> In the absence of test result, I'd do empiric water changes. Perhaps 25% daily until you get some test results. <May be your fish shop will run the tests for you, although you should have your own kits if you want to keep golds in a small setup.>
I am not sure what "general tonic" is. I suspect it is nothing but plain salt. However, in the absence of ingredient list, I would play it safe & not mix meds. I have only read about the Interpet stuff on Goldfish boards. Generally, any antibiotics you put in the water will be ineffective for gold fish bacterial ailment - with the exception of Kanamycin. <FAIK Interpet don't list their ingredients so I have no idea how well it will work.>
Your first order of business is to maintain pristine water condition. Fin rot will not clear in bad water no matter what anti-biotics you throw at it. Once you get your water parameters under control, you may add a bit of <aquarium> salt to help with external ulcers. <About 0.1% final concentration.>
Since the fin rot is set in, you will need to treat it. My first choice would be anti-biotic food. This gets into the fish much better than just having the antibiotics in the water. And you run far less risk of the antibiotic killing your filter bacteria, <which will then cause a mini-cycle & make everything worse.> If you can't get anit-biotic food, you can make your own. This is a recipe:
Goldfish and Aquarium Board Article-Medicated Gel Food
If the fish is not eating, or if antibiotic food is not available, my next choice is to use Kanamycin in the water. I would prefer to treat the sick one in a hospital setup to reduce the risk to my other fish.