Thanks for all the advice ... maybe I should keep to guppies. I used to
have a couple of goldfish for many years. One of them grew from 1.5
inches to about nearly 6 inches (from head to end of tail). He got
pretty large but that's after years ...

As for temperature, it's usually around 20 degree C. or 22. So, it's
not that cold.

I was thinking of tetra because I like the neon red and blue ones. I am
looking at small-ish fish that has blue and/or red colours. So maybe I
will get some guppies with blue tail and some with orange tails.

Does anyone has other suggestions?