Mysterious Khuli Loach deaths

[TREVOR PLECO]

Just trying to wok out what is happening to my Khuli loaches as two out of the five have died within the space of an hour or two. They swim all over the tank not worried about the light, as they usually are and within twenty minutes or so they are stone dead on the bottom. – Hectic, they die so quickly!

I have had them for a year at least may be two. In the past they have put up with great water changes and rough and tough tank mates, as I first introduced them and two young b/n plecos to the tank, 110 G, when it was being used for raising koi fry, so I did frequent/weekly 40% water changes The Koi were finally evicted to larger pastures and the tank was slowly converted to a planted tank without taking the water out and just by adding the gravel and an improved canister filter. The loaches have been in their element with all the new wood décor and rock and much more places to hide than before. Over the last few weeks I have introduced shoals cardinals, various rasboras and a couple of corry cat catfish, while the b/n plecos are spawning at full tilt. All other fish are doing well. Over the week end, I did a 20% water change as the water was getting a bit tea like with the peat in the filter and on Sunday I started a DIY Co2 system at 1pbs, according to my calculations. I checked my water today, the PH is basically the same(7 to 7.5), while the KH has gone form 3 to 4. All other fish seem absolutely fine apart from the Khulies. Are they more susceptible to something than the other fish or what could have happened and more importantly what can I do?? I have been turning the Co2 off with the lights. Has the CO2 somehow taken over the bottom of the tank where the loaches spend most of their time? I would have thought the cardinals and other smaller tetras would have been the first to react if there was a problem. Mystified!

[chefkeith]

I'd say that when you added the new fish you introduced some new pathogens into the aquarium also.

Every time you add new fish, especially fish from different origins, you risk cross contaminating your tank with pathogens that the new fish carry but might be immune to. To reduce risk, new fish should be quarantined and be given prophylactic treatments for the most common parasites.

[Diana]

1 bps on that large a tank is not very much CO2 at all, and I do not think it has anything to do with the death of the Kuhlies. The very minor change in pH, and KH is also so small as not to be a problem.

Ditto chefkeith: Something came in with the new fish, and some of your original fish were more susceptible than others. I have no idea what it might be, though. There are a lot of fish diseases, and not all have distinct symptoms.
Might also be a combination of things: Some low grade problem was going on for a while, perhaps even before changing the tank to a tropical community tank, and then the new fish brought in something that was too much when added to whatever else might have been going on.