Sand in my 130gal tank with brown/white patch in it! HELP!!

Bigabmx21

I have about 1/2 inch of sand in my tank. In one area about 12" long there is brown and white nasty looking growth that I can't figure out why.. I have a clown knife, irridescent shark, nigerian bichir and a Motoro stingray in the tank. I do a 20% water change every week to keep Nitrates right, and clean the sand with my vac. tube every week, and about 4days go by and the same spot gets that nasty white and brown stuff on it in the SAME place.... im freaking out...help me please people. Here are some pictures of the growth.

Attached Images

tank sand.jpg (91.9 KB, 24 views)

scalar

just suck that whole spot out with the gravle vac it might help?

zof

It looks like some weird sort of bacteria growth or maybe something died under the sand, either way I would scoop all the affected sand out and examine it for dead stuff, if there is nothing decomposing there I would just throw the sand away, its probably not something you want in your tank, just make sure to try and get all of it and see if that helps the issue.

I'm sure someone with more experience could help more but in my novice experience that is what I would do.