Now that the tank has cycled, on Sunday we bought 7 White Cloud Mountain Minnows.
All seemed to be ok until this evening where we noticed that one fish stays away from the rest, tends to remain more or less at one area of the aquarium at a depth of about 10cm below surface. It did not eat and when moved around was a bit 'tail heavy'. All other fish are fine. We feed it some flakes and also every other day a small amount of daphnia.

I have checked the water and found:
Ammonia - 0
Nitrite - 0
Nitrate - 80
PH - 8
KH - 7
Temp - 24c

To be safe I have replaced 30% of the water. Do you have any ideas if it is something I am doing or could it just be a poorly fish that I have been unlucky to buy?

Thank you

Fish is dead, yet still would like to know what might have gone wrong.

Thanks

Sorry to read about the WCMM. It may well be that the stress of transport / moving, etc, was all too much for him/her.

Your nitrates level is extremely high, though - I'd do daily water monitoring and changes to get it down to and keep it as close to 20 as possible (or no more than 20 above whatever your tap water nitrate level is), otherwise the other WCMMs are going to struggle.

Will do.
Thank you.

Did as you said and since last week have twice changed the water, 8Ltr at a time (30ltr tank) adding 1 - 1.5Ltr R.O water each time, last night another one died showing the same symptoms (noticed them yesterday afternoon, in the morning all fish were ok).

Water parameters were:
Ammonia - 0
Nitrite - 0 / 0.25
Nitrate - 40 / 80 (using API kit, the colour code for 40ppm and 80ppm is the same!).
P.H - 7.5 - 8.0

Had enough of the API rubbish so this evening bought a test kit made by Tetra, tested the water Ammonia Nitrite and P.H were the same as yesterday's, the Tetra kit showed a 25ppm of Nitrate while the API still shows 40 / 80ppm.