Sudden death mystery

[Mear]

I've got a mystery on my hands, and I'm not terribly optimistic about figuring out what happende, but perhaps some of you have some good suggestions.

The day after doing a pretty ordinary water change (20%) in my 260l community tank, I found 7 out of 10 boesemani and 2 out of 3 L236 dead. All the nice, big ones too... I'd had those for about 6 months, had no prior warning, nothing wrong with them, just dead. Unfortunately I didn't think to test ph and gh, because the water here is very clean and stable and only treated with UV light (no chlorine or anything). It's never been off in the three years I've lived here, though it's been raining a lot lately, so I do regret not checking. I also didn't test for ammonia because by the time I found the fish (~20h) there would have been plenty of it anyway. No nitrite, though.

The only new thing I did was feeding them zucchini, but I can't see how that would kill them. But since only the rainbows and l236 died, and not the bolivian rams or corys (who wouldn't have been very interested in zucchini), who knows.

I just wanna make sure I've considered all the options before re-stocking. Do anyone have any suggestions as to what would kill nearly all of those fish, and not the others?

 

[AbbeysDad]

I think I'd sooner question the water quality rather than the zucchini. What is the source? Unknown to you, was chlorine added/increased because of the rains?

 

[flchamp89]

We often think of ph, ammonia, nitrates when these things happen in established tanks. Ph is unfair more times with new fish or established fish with a wc I think of osmotic shock. We often change whatever percentage we think is best. If if we do this religiously every week. Tds builds due to doing the same amount of wc. Plus any top offs. So most inhabitants get used to it but at some point it reaches a threshold where its either too much or the change is too much for the tank.

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[Byron]

I agree it is more likely a water issue than the zucchini (though one can't rule out some chemical substance on the vegetable ??) but we need more data on the water. I am assuming from your post that you do not use any conditioner, so just what is the source water? A 20% water change in a 260 liter (roughly 70 gallon) tank is not much for such a drastic change. Did you notice any odd behaviour right after?

 

[StevenF]

Unfortunately I didn't think to test ph and gh, because the water here is very clean and stable and only treated with UV light (no chlorine or anything). It's never been off in the three years I've lived here,

If the UV lamp owned by the water utility or is it part of your home. Does your water come from a well streem or reservoir?

UV works very well as long as the water is clear. However if the water gets cloudy due to sediment it quickly looses effectiveness. If that happens a utility may be forced to add chlorine as a temporary emergency measure. Another possibility is that increased water flow in a stream my result in increased levels of CO2 in the water and that would reduce PH. Then when you do a water change you fish would have gotten a PH shock. A GH or KH shock maybe a possibility. Those can also rapidly change in a stream.