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Cardinal terras suddenly dead?!

Renzo77

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2 days ago I’ve gotten 6 new cardinal tetras to join my current 5 in a 20 gallon planted tank. I drip acclimated them for around 30 minutes and placed them in the tank without adding any of the bag water in. They seemed fine and got along with the other cardinals. The same day I set up a diy CO2 system that started up the next day.

However this morning I found that 5 of these new cardinal tetras have died mysteriously and were either floating around or at the bottom all belly up. The CO2 seems to have been on last night but only producing quite a small amount of bubbles even though I remember trying to close it last night. I have a sponge filter which I thought would carry on oxygenating the water if such a scenario occurred.

Why could they have died?

I’m going out today but as soon as I get back I’ll post the water parameters.
 

Colin_T

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New carbon dioxide (CO2) units should be monitored very closely until they are working correctly. Having too much CO2 in the tank at night when the plants don't use it can cause fish to suffocate. It can also drop the pH very rapidly if you don't have enough KH (carbonate hardness) in the water.

Without seeing the fish, I would say the new CO2 caused the problem, but the fish could have had issues at the shop.

Where any of the fish gasping at the surface this morning or before they died?
 
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Renzo77

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New carbon dioxide (CO2) units should be monitored very closely until they are working correctly. Having too much CO2 in the tank at night when the plants don't use it can cause fish to suffocate. It can also drop the pH very rapidly if you don't have enough KH (carbonate hardness) in the water.

Without seeing the fish, I would say the new CO2 caused the problem, but the fish could have had issues at the shop.

Where any of the fish gasping at the surface this morning or before they died?
No, the remaining seemed to have acted normal
 

Colin_T

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Check the ammonia, nitrite, nitrate and pH of your tank when you can. Contact the pet shop and ask them to check the pH of the tank the cardinals came out of. See if there is a major difference in pH.

If you can't turn the CO2 off at night, turn an air pump on after dark so it can keep the CO 2 levels down during the night. In the morning, turn the air pump off and the plants can use the CO2 when there is light.
 
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Renzo77

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Check the ammonia, nitrite, nitrate and pH of your tank when you can. Contact the pet shop and ask them to check the pH of the tank the cardinals came out of. See if there is a major difference in pH.

If you can't turn the CO2 off at night, turn an air pump on after dark so it can keep the CO 2 levels down during the night. In the morning, turn the air pump off and the plants can use the CO2 when there is light.
Ammonia - 0.2ppm
Nitrites - 0ppm
Nitrates - 40ppm

My pH is around 7.6-7.8.
My pH before was around 8.2 before but I’m guessing the CO2 has altered it.
Could this be a contributing factor and can’t the bubbles from my sponge filter help with the CO2?
 

Deanasue

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The nitrates are too high. You need to do a 75% water change and keep Nitrates below at least 20ppm but preferably at 10ppm or lower.
 
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Renzo77

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The nitrates are too high. You need to do a 75% water change and keep Nitrates below at least 20ppm but preferably at 10ppm or lower.
Water straight out of my tap is 40ppm so that’s the lowest I can go or I get an RO system. I guess it’s a common problem for people who live around west London.
 

seangee

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That level of nitrates won't kill them overnight (usually but my comments on stress below apply here too). They would probably appear fine but in general would have a shorter life.
FWIW I have always found DIY CO2 systems more trouble than they are worth. If your main focus is fish then just use plants that don't need additional CO2.

Do you know the parameters of the water the fish came from? My LFS sells captive bred cardinals that are kept in regular tap water (I am west of London so we also have very hard water). They also sell wild caught cardinals which they keep in re-mineralised RO water. These probably would not cope with the stuff that comes out of our taps.

Usually if fish suddenly start dying look at what has changed recently. In your case that's the CO2. Your new fish would have been stressed (and thus weakened) from the recent move so less able to cope with adverse conditions. No guarantees I'm right but based on the info my best guess would be CO2 poisoning that the weaker (or more stressed) fish could not deal with.
 

Colin_T

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Ammonia - 0.2ppm
Nitrites - 0ppm
Nitrates - 40ppm

My pH is around 7.6-7.8.
My pH before was around 8.2 before but I’m guessing the CO2 has altered it.
Could this be a contributing factor and can’t the bubbles from my sponge filter help with the CO2?
Any ammonia in water with a pH of 7.6 is going to be an issue for fish. However, the ammonia reading might be from the dead fish. But you want 0 ammonia at all times, especially in water with a pH above 7.0.

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If you have nitrate problems, and lots of people in the UK do, you can get a pozzani filter. The following links have info about this type of filter. It removes nitrates from tapwater.
https://www.fishforums.net/threads/pozzani-filter-update-and-more-qs.448815/#post-3794258

https://www.fishforums.net/threads/...s-using-a-pozzani-filter.448001/#post-3787278

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If there are lots of bubbles coming out of the sponge filter it will drive out any CO2 you put in.
If there are only a few bubbles coming out, the CO2 levels could build up and cause problems.

Most plant tanks that run CO2, don't have much surface turbulence during the day when the lights are on, and turn the CO2 off an hour before lights out.

Unless you have a heavily planted tank and are adding lots of nutrients (plant fertiliser), adding CO2 is not normally necessary. There is lots of CO2 in the air and that gets into the tank. The fish and filter bacteria also produce CO2 continuously and this normally provides a lot of CO2 for any plants in the tank.
 

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