- Aug 15, 2019
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Awesome thank you! Still learning so didnt know any of this.Do you have a skimmer in the back? If there is a skimmer then the O2 cannot and will not be low at night. If there is no skimmer, and the water in the back chamber is too high, there will not be enough aeration since there will not be any "waterfall" effect as water flow from the main chamber to the filter chamber. At nigh when the light is out, the O2 level may drop and cause hypoxemia and kill fish, some or all of them. Invert tolerate these episode with no significant problem.
Problem like this can be seen in tank that does not have a sump and does not have a skimmer. I think the LFS may be right in diagnose your problem. If this is the case , fish will died overnight with no visible problem during the day. Can solve this by lower the water in the filter chamber, or put an air stone somewhere in the tank.