Beside, if you make it a practice to add medications when none are needed you run the risk of developing strains of bacteria, in your tank, that are resistant to it.
This is a
HUGE point. All you are doing with casual use of Melafix is selectively breeding Melafix resistant bacterial strains. And guess what -- when you need to use Melafix, you will find it very ineffective now. Save it for emergency use, and practice good husbandy via water changes, etc., etc.
This is why it is illegal to buy over-the-counter antibacterials in the UK and other European countries. Something I truly believe that the USA could learn from.
As an aside, the number of deaths from staph infections in hospitals rise every year. Staph used to be (~30 years ago) a minor infection that penicillan or one of the other 'basic' antibacterials would kill. Nowadays some strains of staph are resistant to almost everything. The rate of mutation of these bacteria is greatly outpacing the discovery rate of new antibacterial medications. This is a large concern in the medical community. While the fishkeeping contibution is not a huge factor, it does contribute to the probelm, just like the common antibacterial soaps and sprays and cleansers while is maybe a benefit now, may in fact be a huge detriment in the long term. Sure, a great deal of bacteria is fairly benign, but there is evidence that bacteria of different strains will trade DNA, and once the benign bacteria learn to become resistant, those traits can be passed to the more aggressive (and dangerous to us) bacteria. I remember reading an article a few years back wherein a culture of bacteria in a few generations had changed its own DNA to more effectively uptake the substrate on which it was growing. Effectively, the bacteria was taking a hand in its own evolution. The example in the article was and extreme case, but you can imagine how the same thing can happen in the staph infection, or in your fishtank.
And finally, like with a lot of things in nature, having many bacteria in constant compitition is good. This is also known as Biodiversity. If your fish's immune system is healthy, bacteria should never get their foot in the door, so to speak. You do not want any strains of bacteria in your tank to become dominant (like the Melafix resistant one), instead you want lots of different kinds in constant competition with one another. Which is part of the reason why is it a good idea to change the temperature a few degrees every few months or so. Keeping the tank at a fixed temp. is letting one strain optimally grow at that temp. By changing the temp., even a few degrees, a different strain will be the optimal one. (I usually change my tanks temperatures' as the seasons change -- cooler in the winter, warmer in the summer.) And this competition is very much like nature really is.