That is cool suomi. I thought you might be! i didn't want to say it because it would have been a long shot but wow, i am good at guessing. My wifes dad is a microbiologist, he works with animal pharmecicutals company here in nebraska that mostly does animal feed and medicine.
They come in small pellets OP guy (its 7am here i just woke up, sorry i forgot your name) you just buy them but you gotta pay shipping and wait it out, so i mean i suppose you could just go to a fish store and buy erythomycin, it would be alot for one little beta.
Sunomi, are you suggesting that erythomycin is broad spectrum antibitioc for gram negative bacteria? I have heard that people use it alot, but every time i check it says its motsly gram positive.... anyways, i do'nt think he has an active infection, so i wouldnt worry about the bacterium as much as the mold in his tank, just go to a petsore and get some "rid ich plus" or "fungus cure" or of course the methylene, any of those 3 products will cure his problem, but by far the best is the methylene for at least a week, i will post a link to order Terramycin feed if you so choose, but i wouldnt be worried about infections (if it is one it may well be gram positive and the erythomycin should work) anyways i hope you can understand all that.
Antibiotic feed.
Ken's Premium Sinking Pellets with Oxytetracycline 2.5mm 6-10 days
Methylene B.
Kordon Methylene Blue 4 oz. Use duration of Feed treatment at least 7 days
As you can see, the Terramycin is $1.50 and the Methylene $2.95 and their both guarenteed to work for him, hope i have helped you not confused you. Maybe you can call and ask ken to put it in a envelope since its such a small order and shipping wont be much and he could "rush" it. Just keep the water nice and clean til then.
"Betafix" might work if you dont want to wait for shipping,But he may be too far gone, that stuff is pretty weak just tea tree oil. Sometimes you can get to a good "Fish" store and find the methyelne for $6 and some antibiotic food, or at least a packet of 8 for 8 bucks. I would call around you city and ask about what they have for broad spectrum antibiotics and their prices, make an informed decision and NOT waste too much money.
Methylene + Erythomycin would probably work, im sure you can combine that stuff with ANY antibiotic because its cometely diffrent type of dye.
Peep at this too,
Aquarium Medications Part 2 | Antibiotic & Antimicrobial Treatments
not knowing what if any bacterial infection he has, you might not buy any antibiotic or just whatever is broad spectrum and cheap. But if youve had him in dirty water, and also hes been stressed, he could probably use it. (Just dont buy the Tetracycline, its crap and it will also cause a cycle in your tank and make your water brown and frothy like a cappichino)
Good luck with your little beta