Nipped Gills

Hi,
Just got a new axie. He/she is my fourth. However Thea is a special case. I brought her home because she was kinda the runt, she shows slight nipping at the tip of her tail (nothing serious) and her gills (which is what is my concern).
Thea is about 2 1/2" long, she also hadn't been eating, the others were fat though. She also comes up for air often (which I sure is because of her lack of gills).
I brought her home and she chowed down. She's now active, chubby, and alert. I have her staying the night in a "side car tank" with a bubbler, tomorrow she's bring bumped up to a 15 gal with a sponge filter.
I don't want her gills to remain as little nubs. I'm hoping she's still young enough I can nudge her in having healthy and full gills again. How can I do this aside from proper temp, blackworms, and clean declorinated water?

Gentle water flow instead of fast. The added oxygen in the water will mean that less gills are required to breathe and less will grow. I don't have really have experience with my sponge filter yet (upgrade tank just got done cycling!) but I have an extra air flow valve to use in addition to the air pump to turn down the speed if needed.