Adding new fish to a CO2 injected tank

Matthew Simmons

Junior Poster
Jan 24, 2005
Hi, apologies for the long post

I wonder if anyone has had bad experience when adding new fish to a CO2 injected tank or a clever way of acclimatising new fish to what will be a very different environment to that which they existed in at the LFS before purchase.

Recently, after a long period (7 years) of good tank stability my fish population had declined through natural mortality to the point where I thought I would get some more cardinals to add to the 8 or so I had left from the original shoal of 20. I managed to find some healthy looking 1" fish at the LFS and bought 12.

My normal method of acclimatisation is to tip the new fish into a bowl and then drip tank water into the bowl doubling the volume over 1 hour before tipping the whole lot into the tank.

Within 1 hour of adding the fish to the tank after acclimatisation all the new fish were at the surface gasping for breath despite strong pearling from the plants. I guessed that the CO2 was the issue as the pearling would indicate IMHO good O2. I shut down the Co2 and added a power head to gently turn over the water surface. This did improve the immediate health of the gasping fish.

But to no avail - over the next 3-4 days, 9 of the new fish died and so did most of the old tetras althought the SAEs and Corys (about 8 of these in total) were all fine and showed no signs of distress. (All the crypts melted as well and are yet to recover - but thats another story :( ). Clearly the stress of the change in water parameters to the new fish brought out a disease which they then transferred to the older fish, probably velvet.

I have been using the EI and dosing CO2 as recommended on this forum (and elsewhere) by dialing in CO2 until it ceases to have any furhter impact on plant growth. Until this episode my fish showed no signs of distress. But until I solve this issue I am reluctant to add any more fish.

Should I have wound down the CO2 a couple of days before adding the new fish and then wound it back up again over another couple of days. I hate killing fiish through stupidity.

Thanks for any advice anyone.


Tank parameters

75 deg
Fluval external
Barr type CO2 reactor
water around GH7, Kh5, ph 6.3-6.5 (using test kits)
90w lights
dosing as per EI with KNO3, KHPO4, Flourish to Toms regime on this site
50% water change per week


Junior Poster
Mar 17, 2005
Re: Adding new fish to a CO2 injected tank

Hi Matt,

IMHO, the death of almost all your cardinals had nothing to do with CO2 injection. I rather believe it happened because the fish were not able to deal with bacteria/parasites that were present in the tank (and/or brought into the tank by the new fish). Even though each set of cardinals was probably healthy and had a good general immune system, their specific immune system was not capable of dealing with each other's bacteria. This sometimes happens, and it seems you can do nothing about it, because it even happens after weeks of quarantine. Sometimes only the new fish die, sometimes the old, sometimes both.

Because of experiences like those, I keep new fish in quarantine for a few weeks (to make sure they are not sick). Then I start exchanging water (about a quart daily) between the quarantine and the community tank they are going into for at least a week. Upon that, I exchange small portions of filter material (that is where most of bacteria are) and let it run for another week. The main purpose is to prepare the new and old fish's specific immune system for what they are up to. This way they are only attacked by small amounts of bacteria and parasites, so their specific immune system is trained to deal with them. In your case, they were overwhelmed by the full dose of bacteria at once. I have not had any problems since I have been doing this procedure for each new fish.



Lifetime Charter Member
Lifetime Member
Mar 21, 2005
Re: Adding new fish to a CO2 injected tank

Hi Matt,

I'd like to offer something off the wall and revolutionary. You'll probably think I'm nuts but that's OK. I have a radical theory that suggest your cardinals died of ammonia poisoning. My theory goes something like this:

The stress of transport in the plastic bag causes the fish to excrete feces and urine more that usual increasing the ammonia concentration is such a small volume, however they also respire rapidly thus releasing CO2. The CO2 disolves, and in the same way as in the tank, forms carbonic acid thus keeping the pH in the bag low neutralizing the toxic effect of the ammonia by changing it to ammonium.

Once you open the bag (dilution effect of water addition from your tank notwithstanding) the CO2 escapes, the ammonium changes back to ammonia and the pH rises, immediately resulting in toxicity, perhaps not enough to kill them outright but enough to irreparably damage the gills. Over the course of the next few days they drown...yes, that's right - drown...non functioning gill equals drowning.

Furthermore, in my opinion, the CO2 episode was coincidental and it certainly didn't help matters. That your older fish died is also coincidental - weren't they dying previously? That's why you were out buying new fish remember?

How do you like that theory? Radical eh? Well, based on this theory my philosophy is to get the fish away from the transit water ASAP. Once you open that bag the pH starts to rise and it's like opening Pandora's box. I either net the fish or discard the bag water and immediately throw them into the tank. The only condition I have is that the tank water should be warmer than the bag water, which it usually is. I've done this with apistos, discus, tetras with only a single immediate casualty in the past year (yes, it was a cardinal). But that's only 1 loss for about 60 fishes. Subsequent losses during that period have occured due to agression or jumping (hatchetfish have a knack for finding the slightest crease in the canopy).

Don't laugh too hard....