Large waterchange Did I mess up?

Aled X08

New Member
9 Sep 2017
First off sorry if this is in the wrong place,

Yesterday I did my usual 60 - 70% water change, usual cleaning of glass, light hoover of debris of the substrate and a light scrub of hardscape. Added maybe 8ml total or milton spot dosing some pesky bba while the water was draining to make sure any run off got removed or watered down.
I also did a filter clean, cleaning the media in run off tank water reassembled everything and filled the tank back up with de-chlorinated, right temperature water.

Maybe 2 hours after being up and running a lot of my fish were struggling to swim or had already died. I lost 9 fish in total over the last 12 hours. Does anyone have any ideas as to why? Did I do too much of a water change? Or am I missing something? All the plants seem healthy and most of the shrimp (hopefully only lost one so far)

I tested the water after I noticed a few fish struggling everything seems fine value wise bar KH which is one colour to high on the chart.

Specs are:
Aquascaper 900
Eheim 600T
Co2 injected - correct levels
Kessil lighting on it's usual light cycle

If I forgot anything information please let me know.


11 Aug 2017
Cambridgeshire, UK
I think you’ve answered the question yourself, you did everything perfectly, so nothing wrong there, only you can answer that.

Maybe something in your water. A massive one off use of the water board using a pipe cleaning product maybe?


7 Apr 2008
nr Bath
Hi all,
A massive one off use of the water board using a pipe cleaning product maybe?
That would be my guess as well. Even if you use a dechlorinator like "Prime" it could be overwhelmed by a lot of chloramine in the water supply. If you use a sodium thiosulphate based dechlorinator? it wouldn't have complexed any ammonia (from the break down of chloramine). I'm not a tap water user, but if I was I would definitely use a dechlorinator that complexes ammonia, and I would be very wary of big water changes.

If there is water main work close to your house? (or under construction "new build" houses?) then emergency chloramine dosing becomes much more likely.
By Milton do you mean Sodium Hypochlorite?
I don't think that is the reason, but I'm not a great bleach fan. If you reallly think it is necessary then <"Miltons is probably your best bet.">
Co2 injected - correct levels
If the fish were suffering from gill damage, because of the free ammonia, it would have made them more susceptible to CO2, even if it was at the same level as before the water change.

cheers Darrel