If they've lost all the furry parts, gill filaments, there is something wrong. You may clean your water weekly but you still have to rule out your water parameters.
I'd suggest getting the tankwater tested for ammonia, nitrite and nitrates. If you don't have freshwater test kits, then take a sample of tankwater to your petshop and get it tested and ask them to write it down. Don't buy any chemicals if the petshop says you need them.
How much water do you change weekly, what sort of substrate do you have? Also, how often do you feed them and do you clean any waste/uneaten food (if any) and poo up regularly (other than the weekly changes)?