also just found this if anyone was wondering about tetra cycline, lots of koi keepers use it.
Tetracycline kills both gram positive and gram negative bacteria. The mode of action to which Tetracycline works to kill bacteria is that it inhibits protein synthesis which works against both gram positive and gram negative bacteria.
Tetracycilne does work against some gram negative but has a broader effectiveness against gram positive. However it is more effective when used in conjunction with Meth Blue in situations like this, so it was good you bought it. This combination should do a good job, you'll bring this fish back. Add some of the Meth Blue to the tank too it wouln't hurt since you're using the Tetracycline, Dosing will be on the bottle.
The antibiotic will damage the BB any way that's why without a QT tank you're pretty much between a rock and a hard place as far as that goes. You need to do water cahanges between dosages. After your finished treating run some carbon on the tank do large water change and use some media from another thank.
ok great, i done a 25 percent change today and the administered the second dose of antibiotic. the tank is also now so blue you cant see the back! i have a small internal filter already running as an auxiliary water polisher on the 300 so that will be off use too... says treat for 10 days of anti biotic.