Unfortunately 25% per month is not enough. Do you have a water test kit, and can you tell me what the ammonia, nitrite, and nitrate levels are? Do you know what the nitrogen cycle is, and if so, do you know if the tank is cycled?
An uncycled 5 gallon tank needs at least one 100% water change a week to keep the ammonia levels at zero. A cycled tank needs at least one 30%-50% change per week to keep the nitrate levels down to a tolerable level.
Honestly, I can't really tell if that is ich since the picture is a bit blurry--are the white spots flat, or fuzzy? If they are fuzzy, it might be bacteria infecting the fish. If it is not fuzzy it is likely ich.
If the fish has ich, the best treatment is simply clean water. I would do one 100% change a week and one 50% change a week and see if there is any change. Temperature fluctuations caused by nonadjustable heaters are also a cause of infestation--since these heaters lack thermostats the temperature of the tank changes with the ambient temperature of the room. If the room is too cold, these pre-set heaters can't compensate and are often inadequate. You may want to upgrade to an adjustable heater at some point, I use and recommend this one: http://www.fosterandsmithaquatics.co...m?pcatid=11368
Ich, velvet, and other external parasites will go away faster if the water is very warm--usually for treatment I would turn up the heater to about 83 degrees. This speeds up the life cycle of the protozoa, so they die faster.
If the fish starts getting a lot worse, you should consider a malachite/victoria green treatment. I have had some success with Jungle Ick Clear tabs.