Low oxygen?

[Treebeard]

My wife lost a couple of clowns in her 14 gallon biocube which had been doing great, eating well etc. At first we thought it may have been because of a spike in nitrates because she was removing the bioballs a few at a time. I checked the nitrates and they were around 10 ppm which I doubt would be high enough to kill a healthy fish. Talking to another Canreefer last night I began to suspect that they may have died because of low oxygen levels in the water. Just wondering if anyone else has experienced this problem with nano tanks or if its even a possibility? BTW, there is still a goby in the tank and it seems to be fine. It just seems weird that two fish should die at the same time.