what causes fish to suffocate in water?

What causes fish to suffocate in water if the water parameters are good (0 ammonia, 0 nitrites, ~10 nitrates), water changes are done regularly, temperature is within their comfort range, tank is not overstocked, and the water is well filtered and also has a bubble wall for surface agitation?

I had 2 boesemanI rainbows that I got from my LFS about a week ago (one was a week and 2 days, one was 6 days) that started trying to get water from the surface last night. Water change was done on Sunday with prime, and I did another small one last night after I noticed it, hoping it would help, but it didn't. The male died overnight and the female died this morning. She was so desperate for air that she was jumping out of the water before she died. Water parameters this morning are still 0, 0, and closer to 5. GH is 1 drop, KH is 5 drops, pH is 7.4. Was there just something wrong with the fish?! This is breaking my brain. I can't figure out what killed them.

 

What's the temperature of your tank? A higher water temperature would mean less oxygen gets absorbed by the water and vice versa.
Though I doubt suffocation cause this, maybe the temperature, something they ate, something harmful that you might have accidentally added to the tank, try and trace your steps and see if you've done any changes in your usual routine.

 

The tank was at 77 degrees. This was my quarantine tank, which has been running for a couple of months. I previously had other rainbows in it, which have gone on to my display tank. These two were alone in the tank at the time. I do water changes on all of my tanks every Sunday and the others seem to be just fine. I hadn't tampered with this tank otherwise since then, except to feed the fish and two days ago I added new heater clips to the heater, but I rinsed those in tap water and dried them before I put them in, so they should not have been an issue, I wouldn't think, given that they were sold as aquarium heater clips and therefore should have been aquarium safe. I wonder if the male I got 6 days ago had something he gave to the female and this caused both of their deaths? I don't see any obvious gill flukes or anything. His gills look a little swollen and red now that he is dead but they didn't look that way last night, so I don't know if that happened after he died or while he was dying etc. Hers still look normal.

 

How did you acclimate them? I'm thinking the parameters of your water were very different from the store's which could have been an issue if a longer acclimation process wasn't done. Your GH is awfully low as well. What's the pH, KH and GH out of the tap?

 

I drip acclimated them when I got them from the store, for about two hours each, because of the difference in GH. The GH comes that way out of the tap, unfortunately. The pH, KH and GH are 7.6, 5 and 1 respectively out of the tap and don't change much in my tanks during the course of the week. I have seen my pH get down to 7.4, but that is not much of a shift. The TDS out of the tap is around 120 and this tank was running 114 last night, so not a major shift. I don't think it was an acclimation issue because they were well acclimated and the most recent addition to the tank was 6 days previous.

 

Then I would guess it was bad stock. Maybe you could check with the LFS to see if they had issues with the fish.

 

Did you do a water change recently? I'm wondering why your nitrates appear to have dropped.

 

I don't think it was an acclimation issue because they were well acclimated and the most recent addition to the tank was 6 days previous.

It can take upwards to a week for a fish to become fully acclimated to your water, or so I've read. Drip acclimation, and other forms are a way to lessen the initial shock that would kill them quickly. That said, I agree it doesn't exactly sound like an acclimation issue when you say -

She was so desperate for air that she was jumping out of the water before she died. Water parameters this morning are still 0, 0, and closer to 5. GH is 1 drop, KH is 5 drops, pH is 7.4. Was there just something wrong with the fish?! This is breaking my brain. I can't figure out what killed them.

If a fish is in desperation - seemingly, trying to escape - from your water usually there's something in it you're not able to test for and therefore unaware of. Could be bad stock, just something that happened because it does happen. Could be something you added to your water unknowingly like a rock, other decorations, windex cleaning the glass (although I think ammonia might spike??) and on and on...

 

Yes I did one last night when I realized they appeared to be having trouble breathing. Prior to that, it was last done on Sunday, which is my usual water change day. The water change I did last night didn't seem to help at all, even temporarily.

I haven't added any decorations, etc. to the tank recently. Besides the gravel, , bubble wand, filter and heater, it just had a few plants in there for cover and I didn't even rinse them off this week. I used one of those magnetic glass scrubbers on Sunday, but no chemicals to clean, and the scrubber hadn't been in any chemicals.

 

Hmmm... Well, I can see why it's nagging at you not having a cause for the effect. I think el337 is probably correct with bad stock, maybe even conditions prior to your purchase.

You mentioned reddened gills. Could be the tank it came from was spiking in ammonia/nitrites, burned their gills and you just bought them on the downward spiral of things.

Stuff happens.

 

Your GH and KH are low for rainbows, but it would have effected them within a day if it was the case.
Drip acclimation is really only needed if your TDS is considerable different than the system that the fish is currently in. Otherwise, just getting the temperature fairly close and dumping the fish in a net and into the tank is fine. Osmosis or temperature shock are the only things that will kill a fish from acclimation. Fish adjust quite rapidly if they don't die from shock, much like you do when you jump into a cold swimming pool.

I would keep a watch on the other fish closely for parasites, as infected gills could have caused the issues with breathing and would have taken time to grow to a point where it became an issue.

 

Thankfully they were in a quarantine tank by themselves, so the impact is minimal. Heck, I was even still able to return both fish to the store for a refund. It just always bugs me when I lose a fish and I can't figure out why, so having both of them die so close together with this breathing at the surface issue was really nagging at me. I've broken down the tank and rinsed everything and will let it all dry thoroughly before setting it back up. I need time to seed a new sponge in my display tank anyway since this one may or may not have been hosed up with something contagious.

Thanks for the brainstorming, everyone!