There are instructions for injecting fish with kanamycin in case you're interested. I think injections are very effective for serious infections like this and if some fish are in bad shape you might want to consider it. Also, you'd only use a tiny fraction of medication it would take to dose 300 gallons, but you'd have to be very careful about getting the dose right because it's so small.
"Inject 20 mg kanamycin sulfate/kg (= 9 mg/lb) of body weight IP every 3 days for 14 days."
Fish Disease: Diagnosis and Treatment by Eward J. Noga
The book goes on to say that 10mg has been toxic in some cases so I'd be careul not to overdose and err on the low side.
Kanaplex is 65% kanamycin sulfate and the rest is (I believe) potassium sulfate, so you'd need to adjust the dose if you wanted to use the dose suggested in the book.
The measuring spoon is supposed weigh out 180mg. If 180mg of kanaplex is mixed into 1/4 cup (59ml) filled with water, each 1ml or cc drawn into a syringe will deliver a 3mg dose of kanaplex (180 ÷ 59). If 180mg mixed into 3/4 cup (177ml) then each 1ml will deliver a 1mg dose (180 ÷ 177).
The book calls for the injection to be IP (intraperitoneal) but apparently there's some debate about how much it actually matters if the injection is IP or intramuscular.
Here's one site with information on injecting fish. I also viewed a video of this sites author demonstrating IP injection on YouTube which you should take a look at if you plan on doing this.
http://www.fishdoc.co.uk/treatments/injections.htm
