it is because the sand is settling and now is compacting and killing the beneficial stuff if there is only just a small spot you can vacuum the sand and put new water to top the tank up. Vacuum every 3 weeks and it will be OK you just need to get the dead stuff out of there. A sand sifting Gobi will control this in the future. If the area is large you may want to vacuum and them take it out and rinse it and place it back in the tank.

Bill