Black slime on aquarium and decorations

Question
I have a 29 gallon unplanted aquarium with 5 danios and 2 bala sharks, all about 2-3 inches long. It is about 5 months old. I have a 40 gallon h.o.b. filter, which I replace every few months and only give it a quick rinse in tank water once a month between replacement. The pH is 7.6. Ammonia, nitrites and nitrates are all at 0, tested with API Freshwater Master Test Kit. I had a problem about a month ago with a black slime covering everything in the tank. Following the directions of the local pet shop, I manually cleaned the tank with a scrubbed credit card (this stuff came off in sheets) and took out all artificial plants and decorations gave them a diluted bleach soak until the nasty stuff came off. I rinsed them until the bleach smell was gone and allowed them all to air dry to remove any residual chlorine. I removed all the gravel and gave it a good rinsing as well, not too much black stuff so I used no bleach. I left the filter alone except to clean of the chunks of this stuff off the face of it. I also replaced the light at this time (on about 10 hours a day). The tank doesn't get direct sunlight. I hadn't realized what a mess it was until it was sparkling clean and colorful again. We had previously kept our goldfish in it, but they have been relocated to our 1000 gallon pond outside. We weren't sure if we were going to get anymore fish, so we emptied and cleaned the tank. A few months later we refilled and cycled it before we got the new fish. We never had this problem while the goldfish were in the tank.

Now, this black stuff is coming back. It is slowly growing on the tank walls, artificial plants, decorations, and the glass rocks scattered in the gravel. The fish seem to be thriving. Is there anything I can do to prevent this mess or am I doomed to thoroughly clean my tank every month?

Answer
Dear Tawny,
The re-occurance of the black slime algae must mean that the real underlying cause has not been dealt with. Do you care for this aquarium different than when you had the goldfish in it?

According to my research, the presence of algae outbreaks like this is almost always due to a nutrient imbalance. It could be Cyanobacteria (which can be blue-green, brown, or black) Anerobic conditions, or high levels of organic matter. It would be quite ironic if your nitrate levels were truly ZERO. Usually that is almost impossible to achieve except with massive daily/twice daily water changes. Are you certain your test kits are accurate? They might be out of date. Or you may even be getting false readings if you use a water conditioner that neutralizes ammonia, nitrite or nitrate. Be sure to check this.

The algae can be a sign that your maintenance schedule is not enough for this tank to keep everything balance out. The first thing I would be sure to try is more frequent partial water changes. I know its a lot of work, but water changes are one of the best things you can do for your fish and one of the easiest solutions to water quality/tank problems.

Try doing 50% or more everyday for at least 1-2 weeks. Vacuum the gravel extremely well and make sure to rinse the filter whenever it looks dirty (in old tankwater of course)
Try the water changes, along with removing any black algae you can see. It may also be helpful if you had several large bunches of fasting growing aquatic plants to take up any nutrients and outcompete any algae for food. You can try Anacharis, Hornwort, or any other fast growing species.

I don't think a complete tank tear-down will be necessary. It may take a while, but 98% of water/algae/pollution problems can be almost always eliminated by an aggressive water change schedule. Manually remove the algae at every water change also.

Let me know how this works out...
Best of luck!
Karen~