Hi Ian,

1. Parfois and quelquefois are interchangeable.

2. Une mare is smaller than un étang which is smaller than un lac. All to do with depth...

Un bassin is more the kind of thing you will find in a château , man-made to contain water, either with a statue and a fountain in the middle of it  or even a lake as in Versailles, where you have 'le grand bassin et le petit bassin',(geometric in shape) and also known as 'le grand canal et le petit canal.

3. You cannot say pièces de tomate, the word 'pièce' is used for things sold in units but never meaning 'a bit/piece of something'.

On French markets you might see -

'laitue - €2 pièce'  = 'lettuce - €2 each

Incidentally, you can use 'une pièce d'eau' for a large pond. In Versailles there is a 'Pièce d'eau des Suisses', which is a lake really...

Tricky....

Hope this helps!