Do you know your water parameters? If you are only topping up following evaporation then you aren't removing any waste and nitrates and phosphates will just build up and up. Your water will also just get more and more acidic.
You need to be removing water to remove the waste build up.
If thats the case, don't suddenly do a big water change because your fish will have got gradually used to these water conditions and a sudden change might kill them. I would recommend doing very small water changes to start, build up the amount you change slowly over over weeks/months until you are replacing minimum 30% per week.