Question: Nitrates down, no water change

[Tenuto]

Hi everyone, I'm new here and have been loving all the informative threads. That being said, I have a cycling question.

I'm cycling a tank in prep of getting an axie. The tank atm only has a HOB filter and a sponge filter as well as a glass jar inside.
On New Year's Eve (after about 2.5 weeks of cycling) I saw what what appeared to be my Nitrate spike; it was up to around 40-80 ppm. Awesome. Ammonia was down to about .25 ppm so I added more ammonia and went about my night.
Now, coming home from work I tested 3 times to be sure and Nitrate is down to about 5 ppm. I haven't done a water change so I was wondering if anyone had any idea as to what might've caused such a drastic change. Has anyone seen this before?

As a side note, ever since I added the sponge filter I've noticed some black debris along the bottom of the tank. Could this be the cause? I washed the sponge before I put it in but the only causes for this that I can think of would be that the debris is from the sponge filter or from the activated carbon in the HOB filter. Would a water change be in order? I don't want to interfere with the cycle needlessly.

Thanks in advance!

 

[NaterPotater]

Any chance you've got algae growing? Doesn't sound like that's what the stuff at the bottom is, but there ARE nasty types of algae that are very dark and fuzzy. Nitrate is difficult to get rid of even by doing water changes, so I really can't think of a reason it would go away on its own other than big/frequent water changes or something consuming it like plants or algae. High nitrate causes algae to grow.

 

[Kitan]

Yeah, I was thinking similar to Nate that it may be some kind of algae. If you DO have algae, that does eat up nitrates and 80ppm will encourage algae.

What are your nitrites and pH at? Temperature? These things can cause your cycle to crash, which is why I am asking.

Yes, you should do a change. A water change shouldnt mess with your cycle much, since beneficial bacteria live primarily within your filter media.

As a great tip, you could include some bio rings or bio balls after the charcoal in your HOB filter to give the bacteria another place to live