They acid washed our moss rock waterfall and grotto after it was installed (much to my chagrin, I loved the patina!) So if you're not trying to save any of the moss or lichen you can certainly go that route. You might also try power-washing, if that works then you won't have to worry about getting acid all over the place.

Are you pool chemicals at there right levels? I had algae growing on my rocks and it was due to my pool chemicals not being properly balanced. Namely chlorine.
I just scrubbed mine and then shocked the heck out of my pool for three days straight. I haven't had this problem any more.

The rocks were 'acid washed' when they first put in the pool--which was done when I wasn't home. I would have preferred the beautiful moss that was growing on them. That was almost 4 years ago---now, the moss is growing back beautifully--along with some baby ferns in the crevices... :), but this black stuff growing is what I would like to get rid of. It seems to grow more in the shaded areas like from the trees & the tropical jungle we have growing there. P.S.---The gardener found a 4 ft. (in which he also said was 'thick') snake with diamonds on the skin!!! He said it didn't look like anything from around here locally--it may have been an 'exotic snake' that someone had & it escaped--or worse--was just let go!!!! :( With that lush shaded 'tropical look' we all want with our lagoon pools, it attracts some unwelcome visitors!!! Now I'm afraid to go anywhere near the waterfalls!! Yikes! I'll be floating around in our 'lagoon' this summer with one eye open!!! Ha Ha

We have a similar problem, but it's just a dark green thin line of algae right at the waterline. Scrubbing with a brush makes it lighter, but there's still a noticeable stain. I figured if it got bad I would try acid on just that area, certainly not the whole rock.

I have seen moss rock turn all black like you're talking about; it's always the ones in the shade. Not sure if anyone does anything about it, but you could spray it with an algaecide or bleach maybe...

Hotwater9:
Yes...most of the 'black' on the rock is in the shade (of the greenery...). Does anyone know if there is a 'down side' to using algaecide or bleach on the rocks? At least where swimmers would hold on to on the rocks along the one side. Do you put any pucks (or pieces thereof..) in the pools in your waterfalls so nothing grows in there? We have our waterfalls randomly turn on for 30 minutes each day to keep the water moving.
Thanks for the suggestion!!!!

whodini:
Yes, everything is currently in balance and has been since March. I guess a good scrub, then spray, and then shock the pool because of everything that would drip/fall into the pool. Now if I could only get over this fear of another snake lurking back there!!! :0 Ha Ha

Try just adding a gallon of bleach and pour it on the rocks...don't wash it off and leave it concentrated. You can use acid as well but you need to dillute that or it could discolor the rocks. You'll probably have to do this regularly like "weeding".

funinthesun

You're in Texas right? Sounds like you're describing a Texas Rat Snake which are common and non-poisonous and harmless, although they have a reputation for being foul tempered and aggressive. I've seen them around my house in the Waco area and especially along the river. Google Texas Rat Snake and click "images" and ask your gardener if that's what he saw. I put one link below.

As for washing the rocks. I would get a scrub brush and some stone cleaner from Home Depot. They sell it in the aisle with all the tile products. Rather than blasting the entire area, just dip your brush in cleaner and scrub the spots you want to scrub.

Alternatively you could pressure wash the spots you want to clean with just plain water.

What I would try to avoid is getting too much cleaning chemicals washing into the pool itself.

Here is a link that might be useful: Texas Rat Snake

barco: Thanks for the suggestion. So you think the acid could discolor the rocks and bleach wouldn't? Has anyone tried straight bleach? Results?
alaska diver: Thanks for the info--especially about the Texas Rat snake! I will show it to the gardener on Thursday. I often wondered about 'powerwashing' the rocks. I guess my only apprehension would be if the pressure washer could mess up the 'joints per say...) between the rocks on the waterfalls. How strong or solid of a rock is moss rock? Can it stand up to the pressure washer? I like the 'no chemical approach'!!!
Good information exchange...I love this forum! I have read so much usefull information over the past five years!

The acid "could" discolor the rock if its concentrated so you need to dilute it...not saying it would...but if it does, you will be VERY mad. Dilute it 1:4 and don't let it sit there for 30 minutes without rinsing it off. The pure bleach will unlikely due anything to the rock even concentrated....it might make it look a little cleaner but it will be only temporary at best.

Becareful with the pressure washer. Mine is not even a very strong one and it will "discolor" concrete whereever I spray it if I'm too close. They probably have enough pressure to break off pieces of the rock that are "weak" or already cracked as well as pull any weak, cracked mortar off. It probably will be the most effective though at instant results and I don't think you would need too much pressure to dislodge it.

Same as the chemicals. Use with care...do a test of a smaller area that is not very exposed first to get a feel for things.

I see a Texas rat snake at least once every 4-5 months.

BTW, I need to build a water fall, I wonder if I should use the moss rocks.

Yes, you have to be careful with pressure washers. I have an Austin Stone house that I pressure wash to clean and get rid of mildew. Start slowly with a wide spray pattern and work your way closer. The mortar is probably stronger than the rocks themselves Depending on the type of rock you're more likely to flake or rock chips than blow apart the mortar.

I've seen one or two Texas rat snakes in my yard over the past few years. They are impressive snakes. Of course if it was an exotic it was most likely some variety of boa constrictor. Some of them have diamond-type patterns.

Either way, I wouldn't stress about the snakes. The only snakes in Texas you really need to worry about are rattlesnakes and cottonmouths. And you're not likely to find either in a pool. Unless your pool borders a river.

Quote: "Yes, you have to be careful with pressure washers. I have an Austin Stone house that I pressure wash to clean and get rid of mildew. Start slowly with a wide spray pattern and work your way closer."

Austin Limestone is a really soft material and tends to slough. Moss rock is a lot more solid, it can be power-washed a lot more easily than Austin Limestone. We use Austin Limestone on a lot of commercial projects and it is a very pretty material, but there's always sloughed pieces of it all around the perimeter.

alaskadiver: Well I printed the info on the link and showed it to the gardener. He said he is pretty sure that is the snake they found behind the waterfalls! He aksed if he could keep the information to show his crew that it was not posionous...but since it said it could climb some 40 feet up high in the trees---now he's going to have to worry about his guys staring up in the trees while they are working on the various yards! :) You mentionted "if our pool borders a river..." well,... Armand Bayou & Nature Preserve (http://www.abnc.org/, borders our neighborhood!! It is a very large Wildlife refuge and wilderness preserve (2,500 acres by us and NASA). So I guess along with the deer that come to our window, we will have to put up with a snake sometime. We have lived in this house for almost 20 years and have only seen the occasional water snake. Thank you so much alaskadiver, for sending the information regarding the snake!! barco & tresw: Thanks for the 'good' info on the pressure washer. Starting with a wide spray pattern would be the way to go...now to find someone to do it! I'm still a little 'apprehenisive' of spending any serious amount of time around the waterfalls!!! So today was the last day of school for the boys & I have a housefull of swimmers celebrating. Hands off the rocks for now. Thanks again for all the input!!!! Now some time for summer fun...oh to be a kid again!!! :)