Random? Rainbow Deaths

[MattIreland123]

250l tank
2 x externals
Set up 3 years
Weekly 40% water changes
All water tested normal, as good as it could be with our tap water
Temp is normally 25 ish but was 27 yest when problem started
Did have 4 red line torpedoes
4 blue rainbow
3 banded rainbow
2 boesmani rainbow
I clown pleco
3 cory

Some of these fish are small & some full size

We feed good quality food in sensible quantities, last water change was 5 days ago

Last night came home and 2 banded (we've had them 3 years) were dead & 1 dying, swimming upside down & gilling like mad. We tested the water, all fine, turned the temp down a degree & did a 40% water change. This morning 1 blue was upside down and nearly dead whilst the other three are all gulping!

The 2 small bose are fine & the redlines, I am already gutted, what can I do to help the rest of my fish?

Many thanks

Matt

 

[N0body Of The Goat]

From the info provided, my first hunch would be low oxygen levels, as warm water holds less oxygen. Get the water surface rippling lots, if you have not already done so.
 
25C is mainstream tropical, 27C is considered to be high end. As a regular temp, this is not good for Redline Torpedo Barbs (that do well in an unheated tank that changes through 15-25C as the seasons change) and is a little high for Lake Katubu Rainbows that are found in water recorded in the 21-25C range (http/rainbowfish.angfaqld.org.au/Lacust.htm).
 
What are the water stats using liquid test kits?
What is the gH and kH of the tank using liquid test kits?

 

[fm1978]

I'm with O2 levels as well, seems very likely!
 
Get loads of bubbles going and some good rippling action. Temp back to 24/25C too.