Hi. Please don't use live fish as a centerpiece. I have heard horror stories about this....and really, it is not unique or different.
There are many, many things you can do.
Get some "ivy" bowls like these:
http://us.st11.yimg.com/us.st.yimg.com/I…
available at Dollar Stores and Walmart for $1/each.
Put in some blue vase gems like these:
http://www.save-on-crafts.com/vasgemmars…
and float some candles.
OR....do up a centerpiece with the ivy bowls: Layer some brown sand and blue sand (available at craft stores like Michael's and online)....and then push down a votive candle. Very pretty and cheap.
OR...you can do something with brown coffee beans....like this:
http://family.webshots.com/photo/2034747…
There are many, many low cost ideas. Use Google and type in "wedding centerpiece ideas" and you will see tons!
Good luck!