"毎日一パーセント良くなります。" has no grammatical errors at all, but meaning is vague. Does it mean "somthing will get better 1% per each day"? if so, "毎日１％(１パーセント)ずつ良くなります。" ずつ = at a time Here is an explanation of "ずつ". https://www.nihongomaster.com/dictionary/entry/94837/zutsu-dutsu ※We don't usually use "一パーセント", 1パーセント or 1％ is better to use.