Was it a new / recent addition?
How long has the tank been running?
Any changes made recently in your tank?
Any issues with temperature spikes or big swings in water parameters?
Just be aware of one potential issue with reptile to fish tank conversions. The reptile use often has high amounts of uric acid that can get baked on due to the heat reptiles need. One molecule of Uric acid breaks down to four ammonia molecules. Uric acid is not that toxic, though it would cause ammonia test to read a false positive. The tank will leach ammonia for some time.