Yes and yes. It isn't recommended that you put your dead fish in your garden these days, but using the water from a FRESH WATER fish tank would be very good for your plants. There is a fertilizer that you can buy called fish emulsion. It is the waste product from fish and would be the same thing you would be doing. Great observation on your part.
I have a koi fishtank at home; I heard that Indians used to put dead fish in their gardens and it would make the soil better...
So can I put my fishwater in my plants as "water" ?