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Mixing Diamond tetra sizes?

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New Member
Aug 10, 2023
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Chicago 'burbs
Good morning,
I have a happy school of five nearly grown (I think) Diamond Tetras. Two are definitely males, one is definitely female and two might be on their way to being male. Their bodies are not quite as big as a US quarter but larger than a US nickel.

I bought two more whose bodies are smaller than a US penny - about the size of my thumbnail. They're in my quarantine tank with two tiny Siamese Algae Eaters and appear to be doing well. My question relates to how soon I should introduce them to my main tank. Should I be concerned about the size difference? My community tank is 55 gallons and is fairly heavily planted. Should I be concerned about size difference or should I grow the two new ones for a while before I transfer them.

I have a school of diamonds in a 150g and I've never observed any aggression. Some people say they get nippy in smaller quarters. I suspect that with your numbers and tank size you'll be fine. How big is your QT tank?
The new diamond tetras should be fine with the older ones as long as the older ones haven't set up territories. Even then the new young fish should be ok. The only other issue is what other fishes are in the tank besides diamond tetras. Some of the other fishes might try to eat the young tetras, (assuming you have other bigger fish in the tank).

You should try to increase the size of the group to 10 or more if you can. They like company. :)
agree there will be no issues adding smaller Diamonds, and certainly agree you need more than seven. This is an active species, and the interaction among males is well worth having them, but they definitely need 10+. You have the space in a 55g so I would look at 11-12 minimum.
Thanks for all the replies. That's all good news.

Q-tank is 10 gallons though it is filled to about 8 since there is no hood. It's bare bottom with a canister filter and lots of plants removed from the main tank and just floating around. They're looking good so far and starting to associate me with food so I get to see them a bit more.

None of the inhabitants in the main tank are big enough to eat the little ones. In fact, the biggest fish are Pearl Gouramis which have tiny mouths. Next biggest are Yoyo Loaches and Siamese Algae eaters. I don't see any indication of territories among the Diamonds. They're all over the tank. One moment they'll all be zooming from one end to the other in a tight group and the next they've all split up. There's occasional chasing, but it doesn't seem to be too focused. I do think a school of a dozen would be pretty striking but I need to watch that I don;t overstock the tank. The other inhabitants are not yet full size.
I doubt a few more diamonds will overstock your tank. In my opinion the benefits of a bigger school usually outweigh the risks of overstocking. (within reason, of course!) Keep an eye on the yoyos. I've never kept them, so this is secondhand knowledge, but I've heard they can get aggressive when they grow up.
Update here. The two new Diamonds in quarantine looked good so I transferred them to the main tank. They look happy! Their first reaction was "Whoah! so much space!" (10 gallon to 55 gallon) The two little ones are generally within 2-3 inches of each other and generally in the same area as the 5 bigger ones. Aside from hanging together, there doesn't seem to be any interaction among them so things are looking good.

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