haha. im being extra diplomatic.
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save your gst reciepts. go on the ird website and look at the depreciation values for assets and calculate your tax shield. sum those together and discount them at what it costs you to take a loan at, and that is what your assets lose in value at the end of the year - today, and represents the amount you will save in taxes.
remember to do this for any assets including property, plant and equipment.
as far as ROI goes, it will be hard to attribute a set of cash flows that represent people coming to the cafe because of the added attraction of the fish tank, but it would be difficult to say that people would be indifferent to the cafe with a nice well set up tank, and great coffee (I hear the coffee is great).