The gills without the filaments (hairy bits) are known as gill stubs. We have two that looked similar, their gill loss was due to poor water quality - previous owner didn't have the time, due to studies and work, to care for them and they suffered.
Is it dried tubifex worm cubes? They foul up the tank as they disintegrate and aren't really that good for feeding axolotls (a better alternative if you can't get earthworms would be frozen bloodworm cubes - thaw the bloodworm before adding it to the tank - some people use the turkey baster to with a bit of tankwater and suck it up and drop it in the tank for feeding). You can order earthworms online or ask your local garden centre/hardware store (anywhere that sells wormfarms or composting bins - where to get the compost worms as they generally have a list of wormfarmers). Your local council may also have a composting facility and will have worms they could sell (or hopefully give away for free).
How often do you clean the tank? Do you spotclean (ie clean up any uneaten food/waste daily or when spotted)?
Also, where do you get your minnows, do you breed them or buy them from the petshop? If you buy them from the petshop, they need to be quarantined for minimum 30 days, and then preferably breed them and feed the offspring. This is so they don't pass on any nasties and you can ensure your axolotl gets healthy live food from a known source. Another thing to avoid is adding any tankwater that comes from the petshop (this could also contaminate your tankwater).
Take a sample of tankwater to your local petshop and get them to test for ammonia/nitrite and nitrates and mark down the figures next to each. You don't need to buy anything to fix it, so ignore the petshop people if they suggest anything. Basically, if ammonia or nitrites are above 0, you need to do frequent partial waterchanges to bring the levels down (every day or every two days).
Being quite active could be due to the warm temperature - what is your tankwater temperature? Generally as they mature and during warm temperatures they are more active but during cooler temperatures, they tend to get become less active, unless food's involved!