thanks for the fast reply

Thanks for reporting this issue! I hadn't even considered that the colour-changing would affect the shoaling trait. I've just pushed an update changing the Japanese Dace so that it does not require a shoal, but will gain extra prestige if multiple are kept to fix this issue. Unfortunately there is currently no way to make other species count towards the shoal size, so you'll need at least 3 of the same colour to get this bonus prestige, but at least you won't get negative prestige from not having three of the same colour.

question about japanese dace, every three days they will change from grey to red and vice versa

but the problem is, they will only change colour if they are happy, which also requires at least 3 of them in the same tank, and the game counts the different colours of japanese dace as different species, so I tried buying 6 at the same time, four became red while the remaining 2 didn't change because the grey shoal was now insufficient

is this supposed to be how it works?

You are a legend!