Foul smelling after cleaning

Question
I have a 125 Gallon fish tank.  Most of my tropical fish are 4 and 5 years old.  Very Large catfishes, kissing fish's etc.  Every couple of month I vacumm out the gravel and take about 1/3 of the water out.  After I refill the tank, the tank smells really bad.  I do not know if it is the chemicals <stress coat> and <water conditioner>. It normally goes away after a few days..but am curious is to why it smells so bad.

Thank You
Kathy

Answer
Hi Kathy,
This is purely speculation, but I think it could possibly relate to the amount of time between gravel vacs.  Ideally you should change 50% of the water each week, while gravel vaccuming 1/2 of the gravel in the aquarium. By doing it so far apart, you allow a lot of detrius to settle in the gravel.  When you vac, it only removes a certain amount, but the remainder is stirred up, this gives a foul odor.  Try upping your water changes and gravel vacs to weekly and see if that helps.  Good luck,
Jamie