Hi.
We are completely new to aquariums and have made the mistake of only visiting this forum after we set up the tank and our 9 year old excitedly picked her first fish.
We had four platys and two died overnight - the nitrite and Amonia levels seem fine but the two remaining fish are constantly at the top of the tank which o have read is due to lack of oxygen.
I have checked that the filter is working and down a 15% water change this morning adding the bio and aqua start as dictated on the bottles but no real change.
Is there anything else I can do to try and save the last two fish and avoid a completely heart broken 9 year old deciding she never wants any more fish ?

Thanks to anyone who can offer any help or guidance !

Is the filter, and specifically its ouput nozzle, positioned in such a way that the outflow of water skims the water surface? That should help ensure the water is optimally oxygenated.

Have you used a dechlorinator?

This might be helpful, should you need it at any stage over the coming months:
https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm
...and these more generally:
https://www.fishkeeping.co.uk/articles ... rates-in-the-aquarium.htm
https://www.fishkeeping.co.uk/articles ... g-essential-equipment.htm
https://www.fishkeeping.co.uk/beginners_articles/

Thank you for replying - no I have the filter quite far down in the tank as that was what the instructions said - perhaps I need a different filter ? I have an aqua flow 50. Another fish died so we only have flame now ☹️

:,(

Can you see a ripple across the water surface? If not, I'd definitely elevate the filter so that either its outflow pipe, or the spraybar if you're using that, skims the water surface to oxygenate it. I would use the spraybar if I were you - I've had fish find the standard output too harsh, causing them to position themselves in the quietest/lowest-flow area of the tank, so worth a try in case that is a factor.

Presumably you're using a dechlorinator? This is crucial.

Do you have hard or soft water? Your water company ought to be able to provide that information for your postcode (although if you get limescale in your kettle, then your water is hard). Platies like hard water. If you and the fish shop are located in a soft-water area, it's possible they were using something to harden the water, then the fishes will have endured a shock due to finding themselves in soft water.