Black/purple dots of algae?

[Billsgate]

Hi,

My tank had been set up for about 7-8 months now. I've first noticed this algae about a month ago. It looks black, but when i like really close it has a purplish shine to it. It is appearing on my driftwood and on a coconut shell i have in there. It's not growing on the glass. They're all small dots.
I have 15W of lighting over this tank (20 gallon). 0.75w/[acronym:82869f63fb="Gallon"]gal[/acronym:82869f63fb]
I add no ferts and do 10% water change twice a week.

Anyone knows what this is? Is it harmfull? if not, i'll just let it be, it isn't annoying me yet.

[acronym:82869f63fb="Thanks in Advance"]TIA[/acronym:82869f63fb],

Thomas

[TankGirl]

I am suspicious of blue/green algae, or cyanobacteria. Any algae that has a "sheen" to it is likely to be it. Does it peel off in a sheet? You may not be able to tell if it is just small dots, but if it is on removable decor I would take them out and scrub them in a weak bleach solution. This can take over the tank and the only real way to deal with it is to treat the tank with antibiotics. Most of the time this won't cause a cycle but it could happen. It is more likely to grow on the substrate and on rocks and stuff in the tank, and less likely to grow on the glass, except in between the glass and the substrate.

[travis simonson]

If you rub some off with your fingers and smell, is it absolutely foul-smelling? That's my blue-green algae test. If it has a very potent, almost putrid smell, it's BGA. Man, I hate that stuff. Hard to tell without seeing it though

[TankGirl]

Simulpost, Travis

[Billsgate]

Thanks for the replies so far.

Is blue/green algae really that colour? if so, it isn't that.
It doesn't really have a sheen to it, it's just that from afar it looks black, but when you look through it to the light it's purple (makes any sense?).
Also, the texture is a bit like velvet, it has the same look.

[travis simonson]

Originally Posted by TankGirl

Simulpost, Travis

Heheh TG - Jinx!

It's really hard to tell because there are so many variants of BGA and, really, all types of algae. I've just learned to go by smell first with algae. If it doesn't stink, then things aren't too bad. And BGA is not really blue at all, the color varies considerably, but is usually greenish to some degree.

[malkore]

BGA can actually appear as blue/green, blackish, brownish, or even maroon. In saltwater tanks, it's always called cyanobacteria, and is black or red.

If it's short, doesn't stink and looks like velvet, it sounds like beard algae...which in my experience has been almost as difficult to get rid of as [acronym:bc1fc9d594="Cyanobacteria"]cyano[/acronym:bc1fc9d594]. Its usually caused by high phosphates.

[Billsgate]

How do i reduce phosphates? What causes high phosphates?

[malkore]

water changes lower phosphates. over stocking, over feeding, and phosphates in tap water will increase phosphates...as well as decaying plant material.

you probably need to get a phosphate test kit first. and nitrate test kits.

your phosphate might be at a reasonable level, but if your nitrates are low, that causes an imbalance, which results in algae.

[Billsgate]

I do 10-15% water change twice every week. I do not have a phosphate testkit yet. I'm going to buy a decent test kit soon. Is Tetra a good brand? It's the only brand i've seen at my [acronym:de3dfaa91e="Local Fish Store"]lfs[/acronym:de3dfaa91e].
I've had a Ludwigia something. It's leaves were completely covered in a thick layer of algae. It was that way for about 3 months, and it lost allot of leaves during this time. I tried to remove them every time i did a water change, but i probably had too much plant waste layong around in my tank. I just got rid of the whole plant yesterday. Going to replace it by Hornwort or another fast grower.
So i'm guessing i had too much waste and my tank was slightly overstocked. I moved my Chinese danios, so it's no longer overstocked.

Also, on a side note, i had a look at the hardness of my water (looked at it on my water company site) and it was 37°F or 20.7° dH. Is this hard? What influence does this have on water stability and plants (or fish) to keep? I filter over turf and charcoal, does this do anything to my hardness?

Sorry for the 'hard' questions.