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I'm thinking of building a plywood and glass fish tank in a cabinet from an old television. I plan to put a sump on the back, or maybe on the side behind the channel changer. I want to have a panel of slow moving water over some fabric that's under a light 7x24 to grow algae in the sump to export nitrates.

I plan to put cheese cloth (a food grade fabric) on a panel under the input to the sump, and promote algae growth on it by keeping it well lit. Realizing that this is a little bit unorthodox, since very few people encourage algae growth anywhere near their freshwater aquarium, I have a few questions about the nature of algae.

Does algae stay put? If I grow algae in the refugium with a mechanical filter between the algae and the pump, will the algae grow out into my tank? I could construct the refugium so that the light would be constrained to shine only on the algae panel, and keeping the light out of the tank is trivial.

Does algae incorporate a lot of nitrates when it grows? I know that algae needs nitrates and light to grow, but it's not so clear to me that it actually takes a lot of nitrates out of the water when it does grow. Does anyone have any evidence or data that it does? In short, will this work at all - can I export nitrates by throwing away algae?

Why on earth would you ever do this when water changes are an effective way to control nitrates in a tank? Okay, this one's rhetorical. In fish-supporting natural environments, NO3 is rarely found in concentrations over 5ppm, but in fish tanks, the concentration is often double to five times that. If it's possible to make a system that keeps nitrates near levels seen in the natural environment, at a cost of touching a little algae every now and then to throw away a yucky cheese cloth, I'll take that trade.
 

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I would think it would end up in the main display tank. What works really well for removing nitrates especially in a sump like you wanna do is have part of it being a veggie filter. Grow a fast growing plant like salvinia,watersprite,water lettuce etc. Would be much easier to maintain and whatnot than algae.

Although I believe the theory behind your idea to sound great but I can't see it staying outta the main tank.
 

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I'll just add my limited experience... A few years ago I ended up with a bunch of green algae of some sort, maybe hair algae, growing in just one spot in my turtle tank. It actually looked really cool, growing on an underwater platform, and got 1-2cm long. I went to do a water change (overdue by at least a month), and I was surprised to find the nitrates were at 0!
Then, foolishly, I decided to clean off the platform, but the algae never grew back.

So if algae can keep nitrates nil in a full grown turtle's tank, then I bet it can do the same in your fish tank :) What's more, it only grew in that one spot, on the egg crate platform. Didn't spread to anywhere else in the tank (must have been just the right light in that location).
 
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