What killed (some of) my fish?

[AshRolls]

Ok,so here comes another guy with a new tank, assuming he has a grasp of the whole planted tank game after reading the forums extensively, who suddenly hits reality and loses some stock. Any help would be appreciated so I can clearly see where I went wrong.

New planted tank (180L rio) (amazon sword, dwarf sag, moss, ludwigia etc)
Liquid carbon dosing (5ml = 1.5x recommended dose) (neutro co2)
EI dosing dry salts directly into the tank
Cycling for 5 weeks, tested for 0 ammonia and 0 nitrites

I introduced 12 cardinal tetras, 6 cherry shrimp and 2 amano on sunday and all looked happy.

Last night (Tuesday, 48 hours after introduction) I got home from work and dosed the usual 5ml liquid co2, dosed macros, and fed a couple of flakes.

20 mins later and 1 cardinal dead, 60 mins later and another 3 cardinals are dead. The cherries seem fine except for one which was looking peaky even before released into the tank. The amanos are also fine.

The remaining 8 tetras look fine today.

Here are my guesses as to what could have happened :-

1) Liquid co2 overdose. The glutaraldehyde got to toxic levels and poisoned the tetras. This is the most likely candidate but I would have expected the cherries to bite the bullet first?
2) Injecting the liquid co2 into the tank caused a LOCALISED toxic cloud before it was fully dispersed into the water column, the 4 tetra that bought it just happened to swim through this cloud.
3) The stress of the lights going on, the lid being opened, the dosing and the feeding knocked the weaker tetras out who were still acclimatising to the new tank.
4) The test kit lied and my tank isn't finished cycling (nitrites and/or ammonia still present)
5) I noticed the tetra trying to feed on the EI salts as they were drifting down to the bottom after dry dosing. This salt somehow damaged them.

Any feedback appreciated, thanks Ash

 

[Andy Thurston]

I'd say weak fish, especially if one was looking peaky before they went in and armanos are ok. Maximum dose of easy carbo is about 1.75x for shrimp and a bit more for fish. Im not sure about dry salts as i have never used them. Cardinals can be sensitive to rapid changes in water quality, captive bred ones more than wild caught. Did you test the water from the shop

 

[AshRolls]

Well it was one of the cherry shrimp that was looking peaky, all the cardinals seemed healthy and happy (squaring up with each other and generally behaving naturally) until the 'event' last night. I was expecting to lose a couple of cardinals due to the stress of new tank, but it seemed suspicious to lose 4 in a short space of time. I guess if they were on the edge then all the stress of the tank activity would be enough to tip them over.

I didn't test the water from the shop, though it is only 5 miles away. We live in a soft and very soft water area with high alkalinity due to water company buffering.

 

[Andy Thurston]

The cardinals could be recent stock and may not have been quarantined for long enough at shop cherries may have been there longer too. Out of the five shops i go to most often i only buy livestock from 2. I always lose new fish if i add from the others and i put it down to their water being different to mine. Even though the fish in all 5 shops look very healthy, active and with good colour.

 

[sciencefiction]

Normally it's best if you stop injecting CO2 prior to introducing fish as they will not be used to those levels and also the new water parameters. It's not normal to lose new fish, just make sure you acclimate them properly. I use drip acclimation, also not using the bag they came with to drip acclimate due to the lack of surface area, and I don't float the bag either prior to drip acclimating. It's pointless and just causes more stress.
It's possible that the ones that died were just the most sensitive to higher levels of CO2 for example, or had harder time adapting to new water parameters. Fish differ a lot this way.