Cement rocks = algae = bad?

[csmfish]

A poster mentioned he had a tank and used cement, and, that was the only place that grew algae. This makes me wonder, is cement DIY rock a good idea if it grows algae???

[ngarcia11892]

i think cement acts as a fertalizer cuz iv seen my father put bags around mango trees and they end up looking better then the trees with out it not like a block but just the powder. could it be possible it contains phosphates or anything else unwanted

[csmfish]

Hmmmm, that would be absolutely not cool then with all that DIY rockwork.

[Palting]

I don't know, man. True reef rock is always covered in something. If i'ts not coral, it's algae that covers it. I have yet to see a coral reef that is dead white when I scuba dive or snorkel. And that is what white rock in an aquarium is telling me: it's dead. So for me, algae on the rock is natural, real, and means life. The key is to keep it under control.

[dzfish17]

I have alot of concrete and dry rock from marcorocks in my tank and the algae has spread evenly over both in the last year. The concrete isnt made into rocks but does fill in the gaps between the rocks.

[pryorrods]

After quite a few years making many tons of rock, I've noticed that the type of sand used to form the man made rocks (the mold sand) can be a BIG culprit in algae growth over the long term. When the rocks are taken out of the sand after hardening, that sand is bonded to the surface of the rock. It is always there, even if you try to scrub it off, and the type you use is pretty important for the algae reason alone. Play sand is the worst for algae, and all purpose is not much better. I began using beach sand (right out of the surf) and never had algae problems again.

[MadReefist]

In Tampa Bay, most of the places anywhere near bridges where you can access (that isn't "redneck beach") has old demo'd concrete dumped everywhere to prevent erosion. All I can remember ever seeing growing on these rocks is green hair algae, and it grows very well on them.

Beyond that, mostly only only barnacles and the type of oysters famous for food are about all that grow on them, although every now and then you can spot some gorgonian branches on pieces hard to reach. Stone crabs thrive on these rocks, especially the ones with clusters of oyster shells on them. Occasionally you can find green pistol shrimp or blue crabs living underneath them, and some snails, but that's about all the life I've ever seen rocks like these support.