Sand not gravel can result in anareobic bacterial growth when sand is approximately 2” deep. Also depends on size of sand grains, look for black streaks in sand. MTS will turn this over preventing of anaerobic bacteria Anerobic bacteria release hydrogen sulfide, a toxic gas. If tank aeration is robust, the chance of hydrogen sulfide poisoning is very low. Adding mts will stop hydrogen sulfide production.
Adding assasians will kill mts over time. Then assasins will die without food source, mts return and it starts again.
Like a complete unknown