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Cycle Question


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Can too few fish in too big a tank kill a cycle? After a successful fishless cycle I then introduced three 1.5 to 2 inch goldfish. Now I only have one 2 inch fish in my 55 gallon by itself after giving two away. In fearing the bacteria were not getting enough ammonia to feed on I took the one fish out and moved it to my second tank. I put enough ammonia into the now empty tank to get it up to 1.0 on the test kit. Sure enough after 36 hours, I'm still getting 1.0. Is the cycle gone or just adjusted to the low levels of ammonia that one fiish produced? Is the cycle likely to come back after the bacteria multiply or am I starting from scratch here?

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  • Regular Member

It's very difficult to know exactly where you are without the other numbers. What are you getting for nitrites and nitrates and pH? The beneficial bacteria do die back to accomodate the fishes production rate yes. So fewer fish= less ammonia= bb die off.

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Nitrites are 0

Nitrates are still 5 as they had been with normal water changes with three fish. pH is 8.2. I have a feeling nitrates would be getting closer to zero with just the one fish and continued w/c's

Is my thinking correct that the cycle doesn't even matter with such a lightly stocked tank as long as regular w/c's are performed and ammonia is monitored?

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If you are seeing nitrates it may be that this is only a bump as the bbs adjust and it will be zero in a day or two. Doesn't sound like a cycle crash is happening from the other readings. It just means you need to w/c the ammonia out. The nitrates + high pH indicate your cycle has not crashed.

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The cycle of a tank, once created, is a thing of beauty - a perfectly balanced cycle of life. That is the wonderful way it works.

You have heard the quote "If you build it, they will come"? The same works for the cycle. In a fishless cycle, if you add a certain amount of ammonia, you will cultivate a certain number of beneficial bacteria. The bacteria population will grow until it is as big as it can be with the amount of available food - the ammonia. (and subsequently, nitrite). If you create a cycle using just a few drops of ammonia, you will get a very tiny cycle - with only a few bacteria. If you create a cycle using a large amount of ammonia, more bacteria will grow for you are feeding them.

When you create a fishless cycle, the amount of bacteria are determined by the amount of ammonia you add into the tank. Then, when you add fish, the numbers of bacteria will automatically adjust to the needs of the fish. If you add a lot of fish - they produce a lot of ammonia and more bacteria will grow to accomodate and process that ammonia. If you put in just one or two small fish, they do not produce as much ammonia as your original cycle was using - and some of the bacteria will die off, leaving only the numbers that will be fed by the reduced amounts of ammonia.

This is NOT killing a cycle, though. As long as you have SOME beneficial bacteria living in your media, they are always ready to have a population explosion given the right circumstances. If you have only one small fish ina 55 gallon tank, the tank will be cycled, just the same. You will get low readings of nitrates - for there is not much waste being processed. Now, add in 4 more fish and you will see a small bump in the ammonia for a day or two, then a small bump in the nitrites for a day or two and then the tank will settle and be balanced again. What you are seeing is the bacteria populations exploding and growing because you have added more food to feed them!

Adding in more media as platform for the BB and perhaps a bit more gph that carry the ammonia/nitrite laden water past the BB laden media will help that BB process the waste all the more efficiantly.

But having one small fish means you have a small cycle. HAving more fish mean you have a bigger cycle. The bacteria can and will respond VERY quickly - within days - of being fed more. :)

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