I personally do not find it necessary to do 100% daily changes when treating for anything besides external parasites. Its a personal choice, but I think daily 50%s with a siphon are fine with one 100% per week, because it will cause less stress. I found that when I moved my male VT to a one gallon tank to treat him for internal parasites and did large water changes, he just got really stressed. I ended up just feeding him soaked medicated pellets on his normal feeding schedule in his normal five gallon tank and it worked just fine.
Clean water is obviously a must with fin rot, but ammonia isn't going to build up that fast in a 2.5 gallon and if it stresses him, it will lower his immune system making him prone to secondary infections.