yes, that is probably the cause of your problems.
Sometimes local tap water can have lots of bad things for your reef, like excess nitrate, phosphate, and other nutrients that could cause excessive algea growth or chlorine, copper, ect that could be toxic to your inhabitants.
If your pet store sells RO (reverse osmosis) water you should use that to mix your salt with. or some grocery stores have RO machines as well, and as long as their machines are properly mantained, that should be fine as well. This is what I do, but I have a smaller tank. Many people with larger tanks buy their own RO filters for their home.
Distilled is also fine, though expensive. RO-DI (De-ionized) is probably the purest water you can get for you aquarium, but I've always found that RO is sufficient.
if your petstore sells premixed saltwater, make sure that they're water source is RO and that route would be fine as well.
At this point, I would do a decent size water change using RO as your water source, and if possible pull out the rocks that have the cyano on them, and scrub it off. Use the old tank water to rinse the rocks before you put them back in the tank.
Eventually, the cyano will go away, but it will take time, and keeping up on more frequent water changes. Make sure you're not feeding your fish more than necesisary, as this will put excess nutrients into the water as well.
Oh.. and test your water... Most petstores do this for free. Its just a good habit to get into, and the best way to understand what's going on with your tank.
Hope that helps, let us know how it goes
