I had the same issue. Fry can tolerate it just fine. Kanaplex is an antibiotic, not a treatment for your tank, that's the reason for the frequent water changes so you should also vacuum the gravel a little each time to actually treat the tank. My columnaris problem involved about a dozen guppies (2 pregnant), 50+ fry, a sailfin albino bn pleco and 6 glofish. Columnaris is highly contagious, fish would show signs of infection within 24 hours of being added to the tank. I finally won my fight with only losing about a dozen fry that may have had birth defects since the mom was on death's doorstep when she had them. The lesson I learned from all of this? Water changes! That's how I avoid all problems now. I got a hose and a faucet adapter. Now I perform water changes once or twice a week depending in how stocked the tank is. I siphon 50% of the water, dose the tank with Prime and fill it back up with the hose! My fish have never been happier!