I change 90 gallons each week by using local ocean water. I run the new water thru socks while the temp is coming up and I add the missing salt. Day 2 I drain 90 gallons and pump the new stuff in. Maybe it's just in my mind but the tanks seem invigorated when I dump in the new ocean water.
Wow. Dont you worry about oil and gas lined coastlines with pleasure boats and cargo ship putting God knows what in the beach waterline???
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