Mine actually shed usually after water changes as well, sometimes just one, sometimes both ... and its not every water change, just everyonce and a while ... i think it actually has to do with adding salt ... seems when i add a little salt, like 2-3 tablespoons every 3rd or so 40% water changes in a 220 gallon tank they shed ... not always, so i cant say its def the salt but it seemed that way this week .... it comes off in streaks and pulls back towards the tail ... could also be sand blowing against their skin, and sometimes it happens after i feed, maybe because of a slight ammonia spike ... i cant be positive though on anything, i just know mine do it as well ... mine have never suffered any ill effects and they are approaching 30 inches, and i've had them for 2 years and they are healthy!
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