Blue-green alage ("BGA") is not uncommon in aquaria but neither is it hard to get rid of. I never see it in new or well maintained tanks, just tanks that are weeks or even months behind on cleaning and water changes. I'm not sure what it is in old water it likes but there's something in there (or not in there) it's awfully fond of.

It seems to me it dislikes oxygen; in areas of poor circulation in a tank is where I always find it growing first.

Left unchecked it can make a large mess rather quickly leaving semi-gelatinous sheets of awful smelling blue green slime over everything. It can kill plants by blocking out the light.

The stuff stinks, literally. It smells to me like there's an ammonia component to it which might explain why some plants get a growth spurt when it's in a tank; plants can more easily use ammonia tha nitrate.

Getting rid of it is easy. Just clean the tank!

What I do it this: first I clean the tank with a diatom filter. If you don't have one you should get one unless you're prepared to do a lot more work and want longer. I filter the tank until it's as clean as it can get then I stick my arm in and stirr up as big a mess as I can make and keep filtering. You may have to backflush and recharge the diatom filter, and possibly more than once. Repeat. Each tims use your fingers to carfully massage the alage off rocks, driftwood, glass, plant leaves - whereever it lives.

By now you should have removed at much algae as you physically can and the tank should lool all of a sudden a whole lot better. But, it's still nutrient soup as far as BGA is concerned and it's still in the tank in a big way.

So, what I do next is drain almost all the water out, down to roughly1 inch (2.5cm) per 50 gallons (200L). Now's your chance to clean up a bit more. I like to wipe the insides of the tank glass with hydrogen peroxide or full strength white vinegar. Neither are harmfull if/when they get into your tank, quite the opposite. Hydrogen peroxide in small amounts simply adds oxygen to your tankwater and vinegar will reduce any ferrous iron to a form plants can use. And it does a great job cleaning off lime deposits off the glass.

Refill the tank. I do it straight from the tap. If your water has chlorine or chloramine (test it!) then add the appropriate water conditioned to the tank before refilling it. Let it sit for an hour with no filtration.

Now do this again. No kidding. Siphon all the water out again, cleaning as you go. Refill the tank.

That should do it. It's really that simple. At this point your tank should have crystal clear water and you should should be hard pressed to find any blue-green alage. If you do see some don't sweat it. As long as you change 50% of your water weekly it'll just die and go away. If you find it grows in one spot fists or you can easily see an area of poor circulation it likes to grow in first then increase circulation to that area. A simple stick-on-the-glass sponge fiter is a very good way to do this.

Note than I use this techniqe in tanks a substrate and tanks without. You'll spend a bit more time at the beginning with the diatom filter in tanks with a sustrate because all of the junk inte the gravel or sand.

But either way if you do this then next morning you can smell your tank and for the first time in a while it won't have that characteristic and awful blue-green algae stink.

To be sure you'll still have some BGA on a couple of leaves, or rocks, but if it you can't smell it by smelling the surface of the tank water you're on the winning side for a change. Simply siphon it off the leaves - it comes off very easily unlike most other alage - and replace whatever ater you took out. This may take you 2 or 3 days more, but eventually you'll notice there's none left.

Regular frequent water changes and proper plant fertilization will prevent its recurrence.