How long does it take to get rid of toxic gravel

Question
I've kept aquaria for 20 yrs, never had a big problem. Last night I did a water change in 30 gal tank because nitrites were high, and apparently turned over toxic spots in gravel. Rotten egg smell, and all fish (5 Colomban tetras that I've had for years) were dead in hours. Now only live plants remain. How to start over, and prevent this disaster? Thank you.

Answer
First of all getting yourself a good gravel cleaner and going over it very well several days in a row, doing a 50% water change each time.  I am not sure what kind of filter you have for canister filters they make a product called Nitrazorb its a "rechargable" pillow that will remove the nitrites.  Also, they have some chemicals you can just add directly to the water Nite out I believe to be a good one.  The next step is a restart.  Add a biological starter like Cycle.  Test your water for Nitrites, Nitrates, ammonia,PH.  You know the drill, then you can start by adding some new fish, start out small with just a few tetras (you can always add more). As for prevention, the best way is good maintainence.  Gravel cleaning should be done at least every two weeks. I know this is harder in a planted set up but for healthy fish keeping it is vital.