I'll take this opportunity to address the continued regulation of catching animals from our oceans. This can be illegal or require special permits. Please do not participate in such activities as it can lead to heavy fines/jail time.
That aside, in terms of staying on topic, simply remove salt water and replace it with an equal amount of ro
water from what was removed. I believe there are calculators that can be searched for to assist with exact amounts removed/added, but after awhile it will become second nature for you. I haven't measured anything out exactly on my just replaced 55 or my now running red sea 500 in years. Never any issues except for 1 mistake that was made in the rush of moving into a new house. You'll get into a groove with it.