Help - Serious Algae Problems In My Tanks

[fishnovice]

I have two outside coldwater tanks on a stand with goldfish and golden rudd. Both are mature and cycled with no ammonia or nitrite problems. They are in the shade but still get a fair amount of light. The glass walls on both tanks were covered in green algae which I scraped off. However, I have rwo different problems now:
Bottom tank - The water has turned green and even after 2 big water changes shows no sign of improvement.
Top tank - Water has remained clear but I have just noticed that tank ornaments and plants are covered in black hairy algae, sort of like 4mm hairs which are difficult to scrape off. I took all fake plants out hoping the algae will die if allowed to dry off.
What is this black stuff and how do I get rid of it and stop it coming back?
Just lost 4 goldfish innthis tank, is black algae toxic and could it have killed them?

 

[RadaR]

Both are mature and cycled with no ammonia or nitrite problems.

Good to hear, however no tank ever has absolutely "0" ammonia or "0" nitrite" otherwise the nitrogen cycle wouldn't work. Which leads me on to my next point.
We now know that algae is triggered by the combination of light+ammonia. It has nothing to do with nitrates, phosphates or iron etc. Therefore, the tiny concentrations of ammonia in your tanks that are so small they dont register on a test kit, are causing your algae. If there was no light then there would be no algae, however your tanks are recieveing a lot of natural light. The black hair algae you see is also because of this excess amount of light. In planted tanks we see this algae when the lighting is too high in relation to the amount CO2 we are putting in. It's countered by lowering the light or upping the CO2. In your case, you can only lower the light somehow.
No, the algae isn't toxic but ammonia and organic waste that trigger/spur algae on can have ill effects on your fish.

 

[fishnovice]

Both are mature and cycled with no ammonia or nitrite problems.

Good to hear, however no tank ever has absolutely "0" ammonia or "0" nitrite" otherwise the nitrogen cycle wouldn't work. Which leads me on to my next point.
We now know that algae is triggered by the combination of light+ammonia. It has nothing to do with nitrates, phosphates or iron etc. Therefore, the tiny concentrations of ammonia in your tanks that are so small they dont register on a test kit, are causing your algae. If there was no light then there would be no algae, however your tanks are recieveing a lot of natural light. The black hair algae you see is also because of this excess amount of light. In planted tanks we see this algae when the lighting is too high in relation to the amount CO2 we are putting in. It's countered by lowering the light or upping the CO2. In your case, you can only lower the light somehow.
No, the algae isn't toxic but ammonia and organic waste that trigger/spur algae on can have ill effects on your fish.
Surely ammonia levels so low they are undetectable by API test ar not harmful?
Moved surviving goldfish into cooler tank indoors. They soon recovered and are doing fine, eating a lot and swimming actively.
Thanks for your advice about light. I'll cover the tank to keep it shaded and cooler too.Will cover tank to get rid of algae and put goldies back when it gets cooler.