I Think It’s Cycled??

So I got a tank 4 years ago. It is a 20 gallon high. I cycled it, added zebra danios and Cory catfish. Everything was great! And it has been great for the last 4 years. A couple fish have died here and there. Though everything was/is thriving. After crossing paths with a sick betta fish, I ended up keeping it. I wanted to make sure my water was super good for it, so I invested in a API liquid test kit! I decided to also test my 20 gallon high, though all the fish were happy (fish include- 5 zebra danios and 4 Cory catfish). My results...

Ammonia - 0

Nitrite -0

Nitrate- 0-5 (seems low)

I couldn’t tell exactly what color, but it definitely seems low for a cycled tank. I added a pic. I don’t know why the nitrate is so low. When everything else is good. any ideas ?

Exactly a week ago, I changed from gravel to sand. Could this be why? Is my tank no longer cycled!!

 

There's nothing wrong with having low nitrates, as long as your nitrites and ammonia are at zero. As long as they are, your tank is probably cycled.
You can retest it if you're concerned it might not be correct.
Is your tank planted? Was this after a water change?

 

Did you really really shake up the #2 bottle for nitrates for at least 30 seconds before adding to the tube? If not you can get a really low test reading.
I do my own testing but an LFS wanted to check my water. They didn't shake the #2 bottle at all and that was the reading they got. When I tested the same water at home before I took the water to them the nitrates were about 20.

 

Oh my gosh! I didn’t at all! It is late now, but I will test again tomorrow, and shake it! Thanks. My own stupidity caused me to freak out.

 

Not only shake bottle 2, but vigorously shake it, hit the bottom against your hand, then when you add it to the vial you have to shake them together for 60 seconds.