Melafix contains oils that can harm your betta if not dosed 100% correctly -- and even then.
Besides, it's just an antiseptic, and probably won't take care of advanced fin rot (which is either a true fungal OR a bacterial disease - most often bacterial).
If someone else is taking care of your fish, tell them this very clearly:
1. Put the fish in a covered cup.
2. Change 100% of his water, making it as close to the regular temperature as possible.
3. Dissolve ONE teaspoon of ROCK salt per gallon (not TABLE salt) in the tank's new water - make sure it's dissolved! Or it could cause burns.
4. -Slowly- add a little new water to the betta's cup, every few mins. Ten mins is more than enough, he'll be cold after 20... (and if you don't have a heater - get one asap? Kind of essential equipment, there).
5. Place betta back in tank.
6. Repeat every second day.
The salt will help to halt the fin rot, and isn't too hard to deal with - medications can be tricky for non-fishy people at the best of times. That should tide him over until you get home and can observe how he does on salt for a week or two. If there's no improvement after week (ie, the rot is still advancing/his health is getting worse) grab some Kanaplex OR Furan-2 and treat him with one of those -- they are proper antibiotics, not just antiseptics.
Last edited by Aus; 08-14-2012 at 02:51 PM.