Previous Questions

Why are nitrite levels slowly creeping up in my new tank set up?

What you are experiencing is very common. Your aquarium is still not 'mature', which means the filtration system has yet to establish a level of beneficial bacteria that are capable of consuming the nitrite in your tank. There are no 'set in stone' rules on how long it will take your aquarium to mature as there are so many factors that affect it. You can help the process by doing the following

✔  Manually increasing the number of bacteria in the filter using a commercial product such as King British Filter Aid +.
✔  Manually lower the nitrite in aquarium by performing a weekly, 25% water change, to take the strain off your filter while it matures. Remember to add a de-chlorinator to the tap water.
✔  Use a gravel cleaner to get more waste out of the aquarium. 
✔  Feed only small amounts of food every other day until the filter has matured. 
✔  Do not add any more fish until the filter has matured.
✔  Remember: Nitrite affects a fish's blood's ability to carry oxygen, so make extra effort to ensure the aquarium is well aerated. 

The 'brown powder' is diatoms in the water taking advantage of the conditions. You should find that they settle down once your aquarium matures.