live sand turning black

Question
Hi. I have a saltwater tank thats been running for 8 months or    so and am starting to see my live sand turning grey in some spots and black in others.I am wondering if its a type of algae or if its something i should worry about. I also have a white gobie that sifts the sand and if he eats it is it going to kill him. I also have others and am afraid there in danger as well. Do you have any info on this. Thank you

Answer
Hi Jeremy. This can be quite normal as trapped food and gasses build up over time in your gravel they can become toxic and need to be released periodically. The bottom "trapped" layer of sand will turn a dark grey-black in time, and once the layer is churned up and the waste & gasses released, it will eventually turn back to the original sand color. When you do your partial water changes you should be turning over your gravel in spots with the siphon and raking your fingers lightly through it to release some of this trapped material. However if you notice that it looks more like a tissue paper like growth on the gravel it may be a form of slime algae(cyanobacteria) that is covering your gravel. This looks very obviously to be algae and I think you would easily identify it as such. As far as your goby eating it this should not be a problem as both of these things are not toxic to fish( with the exception of the gases that may build up). Do a good partial water change and really get down into your gravel with the siphon. This will clear up the grayish color you are seeing in either case. If it returns or looks more velvety or you need more help because you think it may be slime algae get back to me and let me know and I will be happy to assist you further.