Fish are dying randomly
I was wondering if I can get a second opinion or any ideas as to why all my fish are dying. I started my tank back in November and stocked it with 8 clown loaches and 8 peacock cichlids in a 125 gallon. The substrate used was 1 inch of organic miracle grow with 3 inches of black diamond blasting sand. I got the API Freshwater Test Kit and have monitored the water parameters religiously. About two weeks in the tank was completely cycled. However, ever since then, the fish have been dying off periodically. At first it was the clown loaches as I noticed they were flashing (scratching on the driftwood). I treated with PraziPro but that didn't seem to do much.
Just yesterday, I stocked the tank with a royal pleco and to my dismay this morning, I found not only the royal pleco dead, but three cichlids and a clown. At this point, I have definitely crossed out just stock quality issues. I tested the water and it shows 0 ammonia, 0 nitrites, and 20 nitrates. I also have to add that on occasions, especially when someone bumps into the aquarium, gas bubbles do come out of the substrate. I am scared to buy anymore fish and desperately want to save what is remaining. My tank is planted with an assortment of plants that is supplemented with root tabs and seachem excel, flourish, and iron. It also has adequet lighting, yet some of those plants also appear to be dying off. Please help!