Testing for Nitrate

I have a freshwater nitrate testkit, along with a saltwater test kit. I tested my nitrates yesterday with the saltwater kit and they showed 15ppm. I tested today with the same kit, and it showed 0 ppm. Is this normal simply by turning on a protein skimmer overnight?

My question, though, is can the fresh water test kit be used for the saltwater too? It only says freshwater on it while some of the other tests says fresh and salt. If so, that one reads 5 ppm.

The saltwater kit uses a powder, while the fresh uses the liquid drops. I don't know the brand sames. Any advice on which is better?

 

It is possible to get a drop in nitrates from skimming just don't know how much of a change. No idea on which is better since you don't know the brands. Probably equal. Look for an expiry date or manufacture date on containers. If you are 2 years past the date of manufacture time to get a new kit. API have the lot numbers on the bottle, the last 4 are the manufacture date.

 

Oh. 0208. I guess it's time for a new kit. (Yes, I see it's API for my liquid kit).

 

API will work for both.