Unknown reason for random fish death??

Hey guys -- Having an issue with my second school of dwarf neon rainbow fish.

I picked up five from my local aquarium store a few months ago. Three females and two males. After a couple of weeks, one of the males died. He was undersized and the other male bullied him relentlessly. From what I read online, that's normal. Figured he was stressed to death. The other male never let him school with the four.

My issue is what's happening this week -- I just lost two of the females for unknown reasons. The first was swimming near the top of the tank for a week or so, gasping, not eating. She finally died today. Three days ago, a second of the females started doing the same thing. I found her dead today, too. I'm now left with one female and one male. Both acting normal, eating, and show no signs of any problems. Both this male and female are larger than the other two females were. They seem to be growing when the other two never got that big.

My problem is that I have no idea what caused the death. I had another school of them six months ago or so, and they all eventually died off. None together, just one every couple of weeks. It's disheartening.

My water parameters are good. 0 ammonia and nitrite, while nitrate is around 20-30. I do weekly water changes of ~ 30 percent. I don't have any other issues with any of the other fish in the tank. There's a Dwarf Gourami, old angel fish, three glass catfish, three cory catfish, and three otos. All of whom are happy and healthy. Tank is 40 gallons with two filters running. I would think if there is an issue with my water quality, the otos & glass cats would be the first to go? From what I understand, those fish don't have much of at tolerance for poor water quality. Yet they're all happy & healthy. The glass cats are going on two years old now.

I'm relatively new to aquariums. Are neon rainbows notorious for dying off? Are there particular to certain water parameters I'm jus not meeting?

Any help is greatly appreciated. I really love the way these fish look & how they school together. I'd love to go pick up three more females to have the school of five back together. I just want to make sure I'm not doing something wrong. Would hate to get more just to have them die, too.

 

Generally when a fish is gasping at the surface it can be a sign there isnt enough oxygen in the water, or something else is wrong with the water. Does the tank have an airstone? Gasping can also be a sign of gill flukes. Some rainbows and other fish can be sensitive to nitrates, so maybe try keeping them at 20 max and see if that helps.

 

Neon tetras are not hardy fish and they are hard to keep. My survival rate is about 50% and they all die with no apparent reason. It’s a well-known fact in the hobby, something I learned after I bought them. You buy 12, you’re lucky if you still have 6 six months later.

Cardinal tetras are healthier. My survival rate with them is a lot higher. You buy 12, you might have 10 left six months later. Or even 12.

 

Neon tetras are not hardy fish and they are hard to keep. My survival rate is about 50% and they all die with no apparent reason. It’s a well-known fact in the hobby, something I learned after I bought them. You buy 12, you’re lucky if you still have 6 six months later.

Cardinal tetras are healthier. My survival rate with them is a lot higher. You buy 12, you might have 10 left six months later. Or even 12.

OK. This kind of reminds me of the otos. Was told to buy five, hope a few make it. That's what happened. Got these three left & they're thriving.

Just makes me nervous whenever I see a fish die. Head immediately goes to worst-case scenario. Seeing the glass cats surviving perfectly fine usually puts me somewhat at ease.

Didn't realize these fish go. Thought I was in the clear with them after they were fine following the first month. Seeing two go, and so close together, made me upset.

I don't want the two left to get stressed and go for that reason. I will probably try to run out to the store and get some today.

Generally when a fish is gasping at the surface it can be a sign there isnt enough oxygen in the water, or something else is wrong with the water. Does the tank have an airstone? Gasping can also be a sign of gill flukes. Some rainbows and other fish can be sensitive to nitrates, so maybe try keeping them at 20 max and see if that helps.

Should have been more specific. Nitrates aren't usually high. They just gradually increase throughout the week. By Saturday is when they're usually ~40, which is when I do my water change and drop them back down.

 

I agree fish gasping at the surface means not enough oxygen, or possibly nitrate poisining. Do a 50% water change and consider an air pump setup or more surface agitation from the filter. Check for symptoms in other fish. We're here to help.
Other parameters could be the issue such as pH or other parameters. It's important to make sure that those are stable as well.