What is this ? Red rusty annoying stuff

[Rene' . Michelle]

Good evening. This is my 2nd salt tank and I've never had this issue before . Can someone help me identify it. I only have 1 fish, I'm afraid to out anything else in it until it's gone.

 

[edsbeaker]

It looks like the tank is newer, and you are dealing with diatoms which is natural.

Here is some helpful information on what you are seeing, and what you can expect moving forward. Diatoms and other algae and bacteria are explained towards the end of the article.

https://www.reef2reef.com/threads/the-supreme-guide-to-setting-up-a-saltwater-reef-aquarium.138750/

 

[Mr. Mojo Rising]

Diatoms which is the easiest algae to deal with, it usually passes on its own after a week or two once it has consumed the available silicates.

 

[Gumbies R Us]

Diatoms! Just let it be, and it will go away on its own!

 

[EnterName]

Others have already answered, but as you might not be familiar with what is coming next, I just want to give you a quick heads-up:

Diatoms are the first stage of "the ugly phase". Your sand and sometimes the rocks get this brown dust all over them. Very often you will also get brown slime covering everything sooner or later. Those are dinoflagellates. It can also happen to get red slime which are cyanobacteria.

Either way, the next step is the green hair algae phase. Your rocks and even parts of the substrate will grow green hair algae for quite some time. At some point this algae will start to die off and the first coralline algae will appear which is the stuff that gives reef rocks their red/pink/purple coloration. At this point the ugly phase ends.

This whole process can take months (sometimes more than half a year) and pretty much every tank goes through it. The important part to know is that this is normal and you do not need to do anything about it.
This means if you search for whatever is currently visible in your tank and you read horror stories and all those work intensive treatments to deal with those pests: Those are meant for established tanks, not those going through the ugly phase.

Keep NO3 and PO4 low. Ensure alkalinity is more or less stable, and nature will do the rest for you.

If you want to pass the time during which the tank doesn't look very good, I can highly recommend getting a cheap compound microscope and exploring the microcosm developing in your tank.