How many corys can I add to my new tank at once? - The Planted Tank Forum
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post #1 of 8 (permalink) Old 05-16-2020, 10:14 PM Thread Starter
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How many corys can I add to my new tank at once?

I just set up my first aquarium since I was a kid at the beginning of April and am about ready to add fish to it and want some advice about how to do so.

I have a 20 gallon long, dirted low tech tank. About half my tank is planted and almost everything is growing pretty well. The tank has been fully cycled for a couple of weeks, but I was waiting out a diatom explosion and adding more plants, so I haven't gotten around to adding with fish yet. I do have 4 nerites that have been in for almost a month that have been going to town on the algae and seem happy. The diatoms seem like they are on their way out and while there is a little green algae growth in certain corners, it seems like it's not growing faster than my snails can eat it. My water is consistently 0 ppm ammonia, 0ppm nitrite, and 5-30ppm nitrate depending on when I last did a water change. I have an aqua clear 30 HOB filter with a prefilter sponge for filtration. My other water parameters (pH, kH, GH, temp) are fine for what I'm planning to do, so I won't go into them.

I want to add a school of salt and pepper corys (C. habrosus). My LFS is going to get some in stock in a week and I really want to add the entire school at once, about 10 to 12 fish. They should be small, ~1/2" each. I know that adding too many fish at once is a classic beginner's mistake and that it is better to add a few at a time to give my filter bacteria time to beef up, but there are some logistical issues that make that tricky. Can I make this work? I have ammonia I can add to the tank in advance to simulate an increased bioload - would that help? If so, how much should I aim for? If you think 10 to 12 is way too many, what's a more reasonable number? I am at home these days, so I can test daily, do as many WC as needed and otherwise closely monitor the situation.

So what do you think? Go for it or tread carefully? FWIW, I won't be adding anything else for a few months.
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post #2 of 8 (permalink) Old 05-16-2020, 11:26 PM
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You can use an ammonia calculator like this....https://www.fishforums.net/aquarium-calculator.htm ...and get the tank up to processing 3-4ppm of ammonia in 24 hrs and I think you would be ok to add the Corys . Even then I would check ammonia/nitrite daily after you add fish and keep some Prime on hand just in case .

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post #3 of 8 (permalink) Old 05-17-2020, 12:49 AM
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What she said. And 10-12 is a good number. You could safely do 20, honestly--I have 13 c. habrosus and 8 c. pygmeus in a 29 gallon and they're not cramped at all.
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post #4 of 8 (permalink) Old 05-17-2020, 01:47 PM Thread Starter
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Originally Posted by Leeatl View Post
You can use an ammonia calculator like this....https://www.fishforums.net/aquarium-calculator.htm ...and get the tank up to processing 3-4ppm of ammonia in 24 hrs and I think you would be ok to add the Corys . Even then I would check ammonia/nitrite daily after you add fish and keep some Prime on hand just in case .
I can work towards 3-4 ppm of ammonia a day. I was doing 1 ppm a day until a few days ago anticipating this issue but had stopped because I was worried about algae. I can spread out the drops throughout the day so there's no sudden spike. That's probably more realistic anyway. I've got plenty of Prime on hand if I need to tie up some nitrogen.


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Originally Posted by KaylSoftpaws View Post
What she said. And 10-12 is a good number. You could safely do 20, honestly--I have 13 c. habrosus and 8 c. pygmeus in a 29 gallon and they're not cramped at all.
I may add more in the long run, but I am leaving it understocked until I have a plan for what is going in the fully stocked tank. The corys were the only thing I am set on having, so they get to rule the roost while I think it over.

Thanks for the input! I'll let y'all know how it goes.
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post #5 of 8 (permalink) Old 05-22-2020, 09:14 PM Thread Starter
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So, I have been adding ammonia, but the corys weren't available with no availability in sight, so I ended up picking up 8 white clouds. They were on my consideration list anyway and just looked so lively in the store. Now at home they are swimming all over the place and the males are sparring, very fun. There has been no ammonia spike yet; I'll keep watching it. I feel like it's going to be fine though! The white clouds are so tiny and my plants have put on a lot of growth this week. I expect things to get a little out of whack, but it'll be manageable.
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post #6 of 8 (permalink) Old 05-22-2020, 09:32 PM
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As a rule, when I add new fish I change 25%-50% ( depending on bio-load) of water for the next 3 days to ensure there is no spike of ammonia and or nitrites.
Within 72 hours the biofilter should catch up with the current bioload.
Why guess it will be fine when you can ensure it will be fine.
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post #7 of 8 (permalink) Old 05-22-2020, 11:09 PM
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I have 12 hastatus corys and a betta in my 10g, and it's fine. Are you setting up a nano fish tank as the habrosus only gets to be about 3/4" long? I found this site helpful for determining stocking levels.

AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor

The one bit of advice I would offer is to make a plan of what fish you want to keep in it. That way, both stocking levels, and compatibility can be addressed before you buy your fish.
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post #8 of 8 (permalink) Old 05-23-2020, 12:32 AM Thread Starter
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Originally Posted by Discusluv View Post
As a rule, when I add new fish I change 25%-50% ( depending on bio-load) of water for the next 3 days to ensure there is no spike of ammonia and or nitrites.
Within 72 hours the biofilter should catch up with the current bioload.
Why guess it will be fine when you can ensure it will be fine.
I can do that.

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Originally Posted by butchblack View Post
I have 12 hastatus corys and a betta in my 10g, and it's fine. Are you setting up a nano fish tank as the habrosus only gets to be about 3/4" long?
Yup, just nano fish, I think! Probably just the white clouds and something else. I'd like the corys, but I'd have to find some available to me. (Though also don't love that they are mostly wild caught - I'd much prefer tank raised fish across the board.) I might end up going in a different direction when it comes down to it, but compatibility with the existing tank is paramount.
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