Yellow stain on gravel...

[Mouthy]

Hello everyone, something recently popped up within my tank and I've searched through the web for some idea of what is going on exactly. Attached I have a pic of what is going on, basically a section of the gravel is turning yellow (pee yellow) and I have no clue on what to do, assuming I should do something.
The second pic is just my tank as a whole, I don't know if that would help but Ima put it up just in case! And just in case this also helps, I have terras, an amp and nano shrimps, Pygmy corries and some amber/ember fish.

 

[BioChem]

Do you have an external media reactor, by any chance? Many times, if you are using GFO, and you didn't rinse it, it will turn gravel and or sand into a yellow to brown hue. Especially where the reactor output line projects.

An FYI: If you have one, it'll work against your plants (as I see your tank has some plants)

 

[Mouthy]

Do you have an external media reactor, by any chance? Many times, if you are using GFO, and you didn't rinse it, it will turn gravel and or sand into a yellow to brown hue. Especially where the reactor output line projects.

An FYI: If you have one, it'll work against your plants (as I see your tank has some plants)

Nope nothing of sorts. And as for any chemicals that I do put within the tank, I only use Leaf Zone for the plants but i don't think that can do this.... right?

 

[nirbhao]

Leaf Zone is very yellow, but I can't imagine it would fall so concentrated and not dissolve into the water. I'm curious to see what you learn.

 

[BioChem]

Leaf Zone certainly wouldn't do this. What equipment or decoration do you have in that specific location?

 

[Mouthy]

No equipment near it except a 50w heater on the back wall which is about 3inches from the stain. Surrounding this spot are three cholla wood and almond leaves about 2 inches from it.