Ammonia & Nitrites finally 0, but Nitrates didn't rise? How?

In my last question about my cycle, I asked why my Ammonia was dropping to 0ppm rapidly while Nitrites continued to be off the charts and Nitrates stayed at 5ppm. The answers given were that my second bacteria colony wasn't big enough yet, and that all ppm goes somewhere, it never just disappears.
Ammonia › Nitrites › Nitrates

Now, that is why I'm confused. My Nitrites finally dropped to 0! Yay! I dosed with ammonia again and both ammonia and nitrites read 0 the next day, that means it is cycled right? Yay.
However, my nitrates are still 5ppm. How does that make sense? My Nitrites were off the chart, so shouldn't nitrates now be, or at least not staying at 5ppm. Where else could they have gone?? I've redone the tests again and again, same results.

 

Have you done any water changes, or do you have plants?

 

third and fourth answer - the target nitrate is 0-10ppm or thereabouts so you have arrived in an ideal world. Mind you aquarists that have tap water high in nitrates don't get excited until they get up to around 60ppm. fourth answer you have managed to grow anaerobic bacteria that eat nitrate. Some say this is impossible in an aquarium situation but others are actively selling media that will do this. Go figure. I would just see if this continues and then introduce the fish.

 

Have you done any water changes, or do you have plants?

I don't have plants and I've read not to do water changes during the cycle, so I haven't.

third and fourth answer - the target nitrate is 0-10ppm or thereabouts so you have arrived in an ideal world. Mind you aquarists that have tap water high in nitrates don't get excited until they get up to around 60ppm. fourth answer you have managed to grow anaerobic bacteria that eat nitrate. Some say this is impossible in an aquarium situation but others are actively selling media that will do this. Go figure. I would just see if this continues and then introduce the fish.

My Nitrates haven't increased OR decreased since reaching 5ppm. Despite changing ammonia and nitrites, it stays at exactly 5ppm always. So I'm not sure that it's anaerobic bacteria. As for my tap water, there are 0 Nitrates. So, I know for a fact that some of my nitrites got converted to nitrates until 5ppm, but then just stopped there. Apparently they are going somewhere else(?)

Is there another kind of bacteria that eats nitrites but converts it into something other than nitrates???

 

How thick is your substrate? Do you have plants? There may be denitrification going on or you haven’t efficiently cycled your tank.

 

How thick is your substrate? Do you have plants? There may be denitrification going on or you haven’t efficiently cycled your tank.

No plants, only fake silk ones. My substrate is sand and it's only about 1 1/2 - 2 inches deep.

I'm cycling 2 tanks. A 29 gal and a 50 gal. It's happening in both.