A couple of Q's.
1. What filter are you running?
2. how bad is the finrot?
3. what were your water parameters(Nitrite,nitrate, ammonia,ph etc) before adding the maracyn?
4.What are the inhabitants of the tank(how many and what type of fish) and what have finrot?
5. were you recommended maracy2 as well?
It is usual when adding any meds to a tank to remove any carbon filtration as the carbon removes any benefit from the med's. You could have halfed a tablet so you would have the dose for 55 not 50 gallons but it shouldn't make a difference.
the majority of cases of finrot etc are a result of a few things but the main one is "water quality". if the water is of poor quality ie to high a reading for nitrite, nitrate ammonia etc the fish start to get ill in various different ways and cant get better, thats why it is important to do regular water changes, clean the filter and vac' the gravel, also not over feeding. the resulting uneaten food and fish waste builds up the levels of toxicity in the tank water and willl eventually kill the fish if not sorted. It's not the only cause for fish problems but it generally is a factor somewhere although whatever you do fish can become i'll.
Might have been better to ask for some advice before you went out and bought the chem's m8 as their are other things on the market, as we aren't here to sell you anything you may have got something cheaper alternative and tips on what to do exactly, most of us have had this prob before so have tried a few different things so you get more than one "Cure" to think about.
Try to answer my Q's and watch for any secondry infection. We can give you better advice if you try and answer the Q's.