I use only Koralias to move water with my octos (not in the 15 gallon merc tank). Puddles had a small ding on one of her arms once and the only thing I could think of was the Koralia and there have been a couple of other nicks and one arm tip that may have been attributed to them so we are still learning and experiementing but they seem to be safer than other uncovered water moving pumps. I have seen my octos play with the water flow from them and KaySoh used one to help her shed suckers but they are still a use with caution piece of hardware, especially for the smaller octos (Puddles was about the size of a mercatoris but with very long arms).