Mystery of the immovable Nitrate

[Breadbox]

So this is an issue that has puzzled me since the very beggining of reef keeping. Other than the starting 2-3 months, my nitrate level seem completely incapable of changing. It remains in the constant 20s no matter what I do, I was dosing 1 ml of Nopox every two day for a few months, it seem to do nothing. So I stopped completely and the nitrate remain in the 20s.

I changed up the food, the feeding frequency and nothing. I turned off my skimmer for a few weeks, no change and then I turned it back on again, still no change!

This is such a bizarre phenomenon, does anyone else have the same issue, what could this be the result of?

 

[V A R I A N T]

Have you tested for phosphate? Nitrate won’t reduce without presence if phosphate, or through heavy water changes.

 

[Breadbox]

A few things I'd like to add, I am using the Salifert Nitrate testkit, and the colour shown typical indicates a nitrate level of 20-50, closer to the 20s.

My skimmer is a bit weak, it is rated for a smaller tank, but it is on all the time.

My tank is beset by a what seems to be a fast growing black coraline algae which quickly covers everything. As a result I seem to have no soft algae growth of anything.

It begins as brown-green but as it matures, it turns black. It is very fast growing, I need to scrape the glass every week and it makes every rock black while sucking up alkalinity like a hog.
If anyone know what on earth that is, please let me know.

 

[Breadbox]

Have you tested for phosphate? Nitrate won’t reduce without presence if phosphate, or through heavy water changes.

Yes, it is around 0.1.

 

[elysics]

You say you dosed 1ml, how big is your tank? And why didn't you increase the dose until anything happened, nitrate going down, phosphate going down, or side effects of some kind?

If literally nothing visible happened, the dose was too small.

 

[Cell]

I think 1 mL every other day in 32G is a pretty low dosage.

 

[Breadbox]

I think 1 mL every other day in 32G is a pretty low dosage.

No way, how did you know that I have a 32G?
I did increase it a little to something like 2ml every 2 days but I stopped because I was getting a bad case of Cyanobacteria.

 

[Cell]

 

[elysics]

Any particular reason you were doing every 2 days instead of every day? Also doubling the dose all at once instead of ramping up was maybe a bit harsh, were you following a particular regimen?

And how bad were the cyanos, just ugly sand, or covering corals? The former can be powered through, the latter maybe not