Nitrates not building?

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I did a nitrate test 2 weeks ago in my heavily planted dirted tank and got around 0-5ml/l. I tested again today without doing any wcs inbetween, and I still got the same result. I'm assuming my plants are eating all the nitrates before they can build up. Does this mean that water changes are obsolete? Is it enough to simply remove mulm?

I've also noticed a bit of algae on my vals. It's short, green and filamentous. Any tips on getting rid of this? My anubias also got some brown-black very short algae on them that doesn't rub off easily. Advice is appreciated! =)
 
Water changes do a lot more than remove nitrate so you still need to do them.

They remove all the debris from the substrate - assuming you clean the substrate during a water change
They remove other waste products dissolved in the water
They remove hormones etc secreted by the fish

Many years ago I worked in a hospital lab and we tested urine for all sorts of things. Fish excrete similar, if not the same, things which need to be removed from the tank. And there are countless other things in urine we did not test for.
Besides the usual hormones made by all animals, fish communicate by chemical signals. These pheromones and allomones also need to be removed before the levels build up too high.


And adding fresh water is like us opening a window in a stuffy room.
 
Thanks for that insightful comment. How often do you suggest I do wcs? The tank is 120gallon/540l, has an Eheim Classic 2260, heavily planted and a lot of catfish etc.
 
Water changes should be done once a week, at least 50%.
And the filter should be cleaned every 2 weeks. The goo that builds up in there is fish poo, uneaten food and bits of plant. The water flowing through a filter clogged with fish poo is not good for fish.
 
I agree with essjay's posts. Moving to other issues raised...nitrates will usually be low in planted tanks. It is not because the plants are taking up nitrate, but more because they are taking up much of the ammonia as their preferred nitrogen source, and this means less ammonia changes to nitrite and less nitrite to nitrate, so nitrate is low. Which is very good for all fish. All of my tanks run in the 0 to 5 ppm range on the API test, and have done so for over a decade now. The water changes also help keep this in check too, by removing dissolved organics.

On the algae, this is likely due to the light. Intensity is one aspect, then duration. If the intensity is not excessive (for the plant species and numbers), then duration can be adjusted. Less "daylight" [meaning when the tank light is on, that is the "daylight" period] may help. Floating plants are very helpful, and fish like them too.

The black brush algae on the Anubias is certainly due to the light, so floating plants should deal with this. Anubias is a slow growing plant, meaning it needs less light and less nutrients, so it readily develops a form of black brush/beard algae if the light is too bright or on too long.
 
Thanks for the input. I have about 14lm/l in my tank, which from what I understand is pretty low. I mostly have low-tech plants, but I'm also adding a bit of CO2. The lights are on for about 12 hours a day, so I'll try to reduce that time. I have some floating plants that are spreading.
 

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