My Nitrate level dropped from between 20 & 40 ppm to zero WTF is going on all test levels were perfect. Done small water change with conditioned water 2 days ago and it was fine till today.....Anyone know whats going on in there?????

nitrate at zero is not a big deal...of course if you have live plants, the plants do feed off of the nitrate or/and phosphate, but you shouldn't be too concerned, for zero is an okay level...in time it will go back up

water testing & consistency...once you have these two things down pack, you will be fine

feshwater nitrate should be less than 20... less than 40 for sure.
saltwater should be lower, i do believe close to 0. saltwater tank inhabitants are more sensitive.. i dont have a reef tank,only keep brackish water for my mollie tank and spotted puffer tank. just keep it low.0-20

Th only tanks that should have zero nitrates are very heavily planted tanks (as the plants use it) If your tank is not heavily planted then i would be seriously questioning why i got a zero reading!

How old is the test kit? Is it a liquid or a strip type? Are you sure you did the test correctly, not shaking the containers for the correct length of time can affect results for example.