I bought 3 giant danios over 1 year ago and never had any problems with them injuring any other fish to include their own kind. While they are very active, they are not fin nippers in any way. The key to keeping these fast swimmers is a large tank, 200 L or up is a must. Mine school with 14 minnows (swallow tail shiners from a bait and tackle shop, that look a lot like them in shape and coloration) in a 280 L tank. Also in the tank are tiger barbs and several algae eaters of various types. No problems with any of them. Aggressiveness in fish is often caused by cramped quarters. This can compared with your neighborhood. Everyone gets along usually, now let all those neighbors move in with you and see what happens. Fish are not that much different.
I have four giant danios with a male and a female zebra cichlid and four upside-down catfish. The danios are nippy at each other, but not tearing each others fins. I notice that everyone with the problem has three or less. They are a schooling fish, and without the school they go crazy.
My wife and I purchased two giant danios about a week ago. Automatically we found that one of the fish in my wife's community tank were dead, and ten another. My wife has a pretty impressive variety in her tank. Then we started to notice bite marks in the fish that would swim toward the top of the tank. We thought it might be the fire eel that was added in the tank. WRONG! After my wife put in a baby Royal Knife Fish, within an hour the two Gaint ASSASSINS as I call them, tore chunks out of it and reduced it to almost bones. We took them out and put them in our hospital tank. They are more than likely going to be going into my tank with the more aggressive fish. My wife's tank has peace again, and the fish that would hide are back out and about swimming.
I have five giant danios and I bought them because I knew that they were active and somewhat aggressive. This is what I was looking for. I have a Vieja synspilum cichlid and before I bought the giant danios he would only hide. Now he comes out all the time. I think that if you have fish that are large or that can defend themselves than these danios are for you.
I have one giant danio in my tank with 5 zebra danios and 5 barbs and he is the most peaceful fish in the tank. I got him when he was small and my other danios were fully grown, so he was slightly smaller than them at the time, as well as being much smaller than the fully grown rosy barb who is the dominant fish in the tank. I think this helped him settle in without a fight and now he tends to swim around ignoring all the other fish.
Giant danios are schooling fish and thus should only be in a tank as a school. I have found that four should be a minimum, in order to stop aggression. They are best kept with larger fish, but when in medium to large schools can include some like-sized fish or even a little smaller. A school of fish is beneficial to tank mates, it keeps them calmer because a school of fish in the wild indicates that there are no large predators around.
People often make a few mistakes with Giant Danios. If you only buy one or two, they have way too much energy and quickly get bored and start chasing the slower fish in your tank, be they smaller danios or anything else that runs from them. If on the other hand you get enough, say 4-5 or more, they will chase each other all day long back and forth at high speed along the tank (looks great in a big school in a large tank) and they are fascinating and very peaceful fish. They do tend to dominate the feeding times a bit, but they have never hassled any of my other fish once I had more than 3. I have them with guppy fry, neons, cardinals, and all of these fish are fine with the giants, but in the old tank when I only had two they constantly hassled other fish. The rule is to get a school or avoid them.
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