How much water does it usually take to top off every three to four days? Big volume top offs in small tanks could cause a significant salinity swing but I don't know if that would be enough to cause this. Tough one.
Molting problems like @Lasse said are definitely possible too.
That honestly could be it. I’m not sure the actual volume to be complexity honest. I eyeball it and haven’t had an issue since. I guess i will start doing it daily..