NO Nitrates

So, I set up a 55 gallon for a local school with zebra danios. mollies, guppies, kuhilli loaches, cherry shrimp, and neon blue gourami's, the other day the teacher told me that the shrimp were dying. I had them test the water with a master test kit and they reported to following, 0 ppm Ammonia, 0 ppm Nitrite, and 0 ppm nitrate. this shocked me as i know that they have been topping the tank off and rarely doing actual water changes so i asked them to test it again expecting to see nitrates. They retested and sent me a picture of the test and still got 0 across the board, at this point I went down to the school with a new master test kit and tested the parameters and got the same thing twice and got a ph of 8.6. is it possible that without plants something in the tank is eating nitrates? Also, there is a large piece of driftwood in the tank so i expected a lower Ph reading any ideas on what's going on with that?