Novices - shake the nitrate bottle really well!!
I am a fairly new fish keeper and I don't want other beginners to make the same mistake I did. Shake the second nitrate bottle really, really well before testing your water! I neglected to do this and injured five fish and killed one guppy because my readings said the nitrate was low. In reality, it was at lethal levels (I accidentally overfed some fry and the extra food caused the nitrate spike). I feel awful that I did this! I might still lose another guppy because her injuries are so severe - we'll have to see if she can recover. Just wanted to caution those who were unaware.
Best wishes for your fishes!