How Important Is The Nitrogen Cycle For Smaller Tanks?

  • #1
Okay so this may sound like a dumb question but bear with me. So I have a 20 gallon tank with some excellent filtration and small fish like endlers and guppies, few platies etc and it's been three to four months and I always end up with 0 ammonia, 0 nitrites and 0 nitrates.

I'm wondering its because I change the water weekly only once like 30 percent. Everytime I test the water the readins are always zero including nitrates so let's just assume my the cycle hasn't been set yet in my tank (even if it is).

The whole point of the nitrogen cycle I thought is so that the fish don't have to swim in ammonia and nitrites. So if you're doing regular water changes etc and the readings are always at zero then is it really because of the beneficial bacteria or just mechanical filtration alone and the biological filtration is really not doing much? In short is the nitrogen cycle really necessary if you're doing frequent water changes and have a small bio load?
  • #2
I would actually assume your tank is cycled. You may have 0 nitrate readings for a few reasons.
1) The nitrates are being absorbed by any live plants and water changes and stay too low to be read on tests
2) You have managed to grow nitrate reducing bacteria which is uncommon but not impossible. Seachem matrix is sold as a filter media for this purpose. These bacteria are eating up the nitrates just like the normal bacterias eat up ammonia and nitrite
3) The test kit isn't showing the right numbers. This may be because it is old or just unreliable. When I first started using the apI liquid test kit I kept getting 0 nitrates but actually I just wasn't shaking the bottle hard enough

Anyway, as for your question I still think it is important to cycle smaller tanks. Without cycling a tank is going to build up ammonia in between water changes which isn't great for fish even if it is only a small amount of ammonia. Conditions get toxic faster in smaller tanks than larger ones. Also it is pretty tricky to stop a tank from cycling slowly once it has fish in it.
  • #3
I run into issues trying to explain the nitrite cycle to people with 5gal tanks or smaller especially if they say they've been doing water changes every week right off the bat. Essentially, if you do weekly water changes on a small tank with a tiny bioload, you're going to continuously halt and stop the development of the nitrite cycle, and I suppose, if you can keep up with this schedule, then there is little wrong with doing it that way.

HOWEVER, the nitrite cycle is our safeguard against immediate changes in the aquarium. If a fish dies in your aquarium and no-one is around to remove it in a timely manner, in an uncycled tank, that could cause an ammonia spike because the tank doesn't have any precautionary bacteria to remedy the issue immediately. and so, its possible to keep a smaller volume tank uncycled and be alright, but it also opens the door for ammonia and nitrite spikes for any given reason. Its safer to let the tank cycle than it would be to count on your weekly water changes.

I would think it's something with your test kit. Its extremely unusual to have a tank that's been set up to still read zero everything. It's very possible your live plants are consuming the nitrate and that the tank is completely cycled, but if you've been doing water changes since the day you set it up, it's possible that the tank isn't cycled. Hard to say with those parameters.
  • #4
So the nitrogen cycle is happening/has happened if you for instance put your fingers on the inside of the glass and it feels slick. That is biofilm, and it is processing the fishes' waste. In a twenty gallon tank with only a few tiny fish, it is possible that your water change schedule is enough to keep your nitrate levels undetectable. Some people think that the mechanical and biological filter are mutually exclusive, or like to parrot the notion that "beneficial bacteria only live in the filter" which is an incorrect logic step from "MOST of the beneficial bacteria live in the filter"...the reason most of them live in the filter is because bacteria populate every surface in the tank, and presumably if you have a good filter with a sponge (they are not just for removing particles) and/or some specific biomedia in there, your filter will comprise the largest available surface area in the tank. The nitrogen cycle is not a once-around cycle, it is an ongoing life system. The bacteria don't only grow when you reach a measured 4ppm of ammonia and follow this specific little formula. If you take a twenty gallon tank and put one teeny fish in it, that fish will start the bacteria growth. He won't poison himself necessarily in the process because there is enough volume to dilute the waste and the bacteria will catch up before the concentration of ammonia becomes toxic. Then say, two weeks later you add another teeny fish, the bacterial population will grow to accommodate that, and so on. This was the old school safe way to fish-in cycle. Everyone talks like it's some all or nothing thing, and it's not. You can take it slow and not hurt anything. There is a saying in the saltwater side of this hobby that I wish more people in freshwater would use: Only bad things happen fast.
If you have no detectable nitrates, good for you! After three months your tank can't not be cycled if there have been fish in it the whole time. Nitrates are the end product of the cycle, but frequent small water changes will keep them very low, especially if you have plants in the tank. Congratulations on a successful setup! Also, remember nitrates aren't the only reason we do water changes, so keep it up!
  • #5
Yes, you’re getting zero readings because of the bacteria in the filter. Without a filter you won’t get them, unless the bioload is minimal.
  • #6
What do you mean by "excellent filtration"? And how many fishes exactly?

If you can create anaerobic conditions in your filter, then there is a possibility to grow bacteria which will reduce the nitrates. However, it is really hard to balance out a filter this way...

Or maybe using some nitrate-absorbing pads - but you will need to keep up with their change, because once they are full, the unabsorbed nitrate can build up very fast causing inconsistency in the system (algae bloom, sick fish).

If you are matching the pH, temperature, and hardness, you can do 100% water change every day - or even more time a day. Most fish won't be too stressed about it, well they can used to it. But, think about the amount of water you will be using! Does not worth it for a 20 gallon...
  • #7
What test kit are you using? What are the readings of the water you are using for water changes?
  • #8
If you are using the API kit you really have to shake the nitrate bottle number 2 for a long time or you get false readings. I have done this before
  • #9
Small tanks even though less water are harder to keep balanced than bigger tanks. The nitrogen cycle is important for all aquariums.
  • #10
As above mentioned a smaller tank is harder to maintain parameters but it is very doable as long as you have the right stocking, fitration and WC routine its a breeze. As a side note always be sure to test your source water to rule out any false readings using the API or any test kit.

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