I got the game today with the Gem Promo offer and I have to say I quite enjoy it, however, at some point or another my "tenants" will begin asking for a "Simple Biotope" and I have absolutely no idea what that is supposed to be.
The "Simple Biotope" is a piece of furniture from the "Decorations" category.
When you're in the tenants' room, click on one of the stars in the furniture panel to the left to add "Misc" furniture. Then select the "Decorations" category (looks like a potted plant and then the "Simple Biotope" (looks like a little pond with water lilies in it.
Thanks, you're absolutely right, don't know how I missed it!
I had already bought the water-fountain so it didn't occur to me that they were asking for a lower-tier item. Duh.