I bought some fantail goldfish at Wal*Mart (don't laugh!) about a week and a half ago. They are developing black spots on their tails and fins. One only has it on her dorsal. The other has it on all of them. My mom, an ex-fish breeder, says that they have fin rot. The marine specialist at Petco says it's natural. Right now, I believe my mom. They don't sell tetracycline anymore because... I'm not giving anyone ideas. The Petco guy actually told me the details... Drug stuff. What should I use instead? Is it fin rot?