Is my fish tank fully cycled?

  • #1
So I got a new tank and before learning about the nitrogen cycle, I got 4 neon tetras a week after I got it. I planted a few live plants before I placed the fish in. It’s a 20 gallon tank.
I noticed initially my ammonia was at 1PPM, but then I realized I was getting the same result from the tap water.
About a week later I noticed an increase in ammonia to 2 PPM, and slight increase of nitrite to 0.25 PPM. I also remember Nitrate registering 5-10 PPM even before I registered any nitrite.
I continued to test everyday and continued to have elevated ammonia around 1-2PPM range, with nitrite at 0.25-0.5PPM. I did water changes every day about 30-50% of water. A week later, I’m registering 0PPM ammonia, and nitrite spiked to 5PPM plus. I increased water changes for a few days, but I could not get nitrite to go down for about a week. Then yesterday I read about using salt to help fish cope with nitrite exposure. So I added some, and quickly realized I may have added too much because the fish started gasping for air. I did two 50% water changes two hours apart to dilute the salt, and even with those water changes, I still ended up with somewhere between 2-5 PPM nitrite. I noticed an immediate improvement in fish behavior so I decided to stop messing around with the tank last night. I did one more water change this morning, and again registered 2-3PpM nitrite. Oddly I have always registered 10-30PPM nitrate during this whole process (starting a few days after I added fish).
Then suddenly, I get home today and test my water before my water change and everything is ZERO PPM! No ammonia, no nitrite, no nitrate! I even retested just to be sure. I also noticed my water was cloudy and it had an uglier smell. Has my tank completely cycled???

note: When I freaked out last night I was only testing ammonia and nitrite so I think the nitrate may have become very well diluted and didn’t register when I tested everything today.

side note: since I planted the plants, I have noticed 5 snails come out of nowhere, which I read could have traveled on the plants.

  • #2
Hi & welcome to the site. Just a few questions, 1. what are you testing with? 2. how long has the tank been set up? 3. what are you using to treat the water during water changes?

  • #3
The only way to know if the tank has cycled is to continue testing. Nitrites hanging on for a while and then suddenly "disappearing" is common in my experience. 1ppm ammonia in your tap water is a lot. I'd let it set in a bucket overnight with an air stone running before using in the tank.
  • #4
Do a 50%-70% water change. That should bring the ammonia to at least .5ppm. Then test in 12 hours. If ammonia and nitrite are at zero you are probably cycled. Continue testing for a few days.

Because of the ammonia in your tap water, you can set up your tank to be lightly stocked so that you only need do small water changes. This will prevent introducing a lot of ammonia each time you do a water change. Or maybe do 2 partial changes per week.

For example: If you test a cycled tank before a water change you should see 0 ammonia, 0 nitrite, and some amount of nitrate. With your tap water, a 50% water change will result in 0.5 ppm ammonia reading in the tank after the WC. 25% WC will result in 0.25ppm ammonia, which is easily handled by your biological filter, and should not be too much for your fish to handle.
  • #5
If you test your tap water, are you getting an ammonia test close to 1 ppm? If so, your water company is using chloramine as the disinfectant. Chloramine is a combination of chlorine and ammonia. If you use a water conditioner containing sodium thiosulfate, it will break the chloramine bond releasing chlorine and ammonia. It will then detoxify chlorine by reducing it to chloride ion. However, you will still have ammonia remaining. That is not an issue once your tank is fully cycled, but every time you do a water change, you will be adding ammonia to the tank.

Since you are doing a fish-in cycle, you do not need to add ammonia to your tank. Since your tap water contains ammonia, I suggest doing small (10%) water changes frequently rather than large (50%) water changes. Although any level of ammonia in a tank is considered harmful, levels over 1 ppm can even kill fish.

The toxicity of ammonia depends upon the pH of your water. If the pH is above 7.6, most of the ammonia will be in the toxic ammonia form. As the pH drops below 7.6, some of the ammonia will be in the ammonium ion form which is less toxic.

You might consider using a water conditioner such as Seachem Prime to treat your water. That is supposed to detoxify the ammonia for a short period until the nitrifying bacteria start to break it down into nitrite.

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