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Hi, all. I have a fully cycled 46-gallon heavily planted tank that lost some of its largest denizens last week due to a problem with my heater. I would like to add about 20 tetras (cardinal and glowlight) as well as eight zebra otos. I will have to order the fish and thus will save money if I ship them all at once rather than separately.

The only information I can find on adding fish says not to add more than one a month in order to keep the cycle from crashing. I'm skeptical that's accurate.

Does anyone know of a good rule of thumb for how many fish you can add to a tank, and how often (obviously staying within stocking limits) and still maintain the cycle?

Thanks so much!
 

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The bacteria colony can double in size in 24 hours, so I have always used that as a guideline. What that means is the bioload that you can safely add to the tank is a function of the current bioload of the tank. The larger the colony, the faster it can grow. If you have 20 neons in a tank, then theoretically you can add 20 more and have things balanced out after just a day or two. In practice I have found that to be more or less accurate - I've combined two tanks of similar bioloads (adding some 25 fish), dosed daily with prime for 3 days, no water changes and no problems whatsoever.

Of course it's important to have the filtration capacity to allow the bacteria to grow.
 

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Ah. Thank you. That's a great rule of thumb. I keep really much higher-capacity filters on my tanks than what is really required, so that might be a good rule for me. I appreciate the response and help!
 

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In a heavily planted tank with fast growing plants you can add a lot of fish with no ammonia spike.

The plants will consume the ammonia preventing spikes.

I do stop feeding just to limit the initial bio load though.

my .02
 

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In a heavily planted tank with fast growing plants you can add a lot of fish with no ammonia spike.


I do stop feeding just to limit the initial bio load though.

That's wonderful, but what about for every other type of aquarium out there, other than your "heavily planted tank with fast growing plants"?


I agree with using feeding as a way of controling the bioload. For the week prior to adding the new fish you can double up on feedings to build up the colony, then fast the fish for a few days after adding them while the tank balances out. I had forgotten to mention that - thanks for bringing it up.
 
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