Water change causing nitrate/nitrite spike?

[birmansandbettas]

I've had this tank running for 10 months. I do weekly water changes and testing that shows nitrates and nitrites at or near zero. I skim the gravel with the hose for waste, but don't dig too deep because the bottom is solidly planted and there are MTS. This has been consistent with no problems since the tank cycled.

Last night I did the usual Monday testing and water change, then today I came home to cloudy water and a lot of dead fish. The nitrates and nitrites are through the roof.

I tested the tap water, and it's coming up 0 ppm for both nitrates and nitrites. An hour ago I did a 50% change with bottled spring water, just in case. The fish aren't acting normal yet.

I haven't changed foods, or amount of food. I used DeChlor as usual.

Any ideas what I did wrong?

 

[Lupin]

Hmmm...Did you smell the bottom part of the substrate? Any sulfuric smell or smell close to rotten eggs? If so, that's where your problem is I think. Hydrogen sulfide must have been released if you've disturbed the bottom part which may have already been anaerobic and the vacuum failed to suck up the toxic substance immediately during your water changes.