I would make a 'guess' that you're right in saying co2 is killing your fish, but based on my exhaustive lurking on this forum, I would suggest that it's not the ph. Yes, your ph swings, that's fine. My ph has swung from 7s to 6s as I gas and degas. However, co2 itself kills. It'll even kill you/me and all other non plant based oxygen breathing life. You're gassing for 15 hours. Since that's killing your fish, clearly that's too much. You started gassing for 6 hours and that works. You never mentioned if your ph is still swinging. If you're getting a good amount in, I'm inclined to say that it still swings. What did your drop checkers show when you gassed for 15 hours? What does it show when you gassed for 6?
Honestly I haven't checked, I was pretty peeved to put it politely and just stopped checking and was more concerned with the fish so that's what I was paying attention too, I've done a check and I'll see how much it swings in 6 hours and get back to you. My thought was not so much the co2 as the fish at no point were gasping or close to the surface trying to get oxygen so that's why I was leaning towards the pH swing being the culprit.
Your fish is also probably suffocating because your O2 levels may be low. You might need to search on co2/o2 relations, or co2 toxicity. There's something there but I'll leave that one for the experts to answer. Try this thread:
CO2 and O2
I pretty much replied to this above, the fish at no point were going to the surface or showing any signs of distress or oxygen deprivation so I dunno...
I'll try to find more for you but many have suggested that they can push the 30ppm co2 envelope further when they introduce extra O2 into the system. Usually this is done by surface agitation. What do you use to circulate the water and break up the surface?
Here's a link that suggest extra aeration will stabilize your CO2 from going beyond a lethal point.
I'm using the HOB and an air pump to break surface tension. So there should now be plenty of aeration and surface agitation.
Did you ever notice a surface film building up when you used to gas for 15 hours? There's also readings that suggest a surface film building on the water will prevent proper co2/o2 exchange and thus causing things to go awry. Did your drop checker get progressively more green to yellow the following day when you did your 15 hour injections?
Surface scum/protein and CO2 outgassing
There was at no point ever film on the top, due too the HOB moving the water. I have seen what your referring to in a friends Beta bowl though. The drop checker went to green, and than back to blue by the next morning....however I was using the Red Sea drop checker and I've since found that it's a POS and purchased a Fluval with the right Dk4 solution and get much more accurate readings
As for me, I inject co2 on a timer cycle close to my light cycle. I gas for 8 hours. My ph swings pretty vastly. I completely degas at night because I run an HOB to break up the surface film and introduce more o2 into the system. You'll find that some people run an aerator for this very purpose. Some people don't bother. They probably have enough surface agitation from their various filtration systems or koralia units or so on.
I can't run my co2 on a timer right now as the only electric item in the system is the Red Sea reactor http://www.emporioaquatico.com.br/me...eactor_500.png
Sorry that you lost your fish to co2. You have a safe point right now. Seems to be 6 hours. Nightspell is suggesting that you increase it slowly. There's some results from people who introduce their livestock to co2 slowly and sort of build their tolerance. I've not looked into this one but I'll give it the benefit of the doubt. I'll echo the wisdom of the many before me in suggesting that you do it only on days when you're home. Perhaps put the co2 on a timer and add a half hour to an hour and watch your fish closely -- especially towards the end of the injection cycle to see how much you can push it. Add in some aeration somehow and see if you can push it even further. There's a balance to be obtained but only you'll know what that is in your tank. Hang in there. Your cause lies in the co2. The ph only indicates that the co2 was there.
I checked my pH again, 2 hours after I turned on the Co2 and it went from approximately 7.5 to 6.9 in that time.
I'm linking to a video I just took so you can see the surface agitation