Warning there are no 'cheats' to get fish bucks and there are a number of scams involving paypal that do not work.
Tap Fish Tip: Free Tap Fish fish bucks.

Fish Bucks are Tap Fish's proprietary in-game currency for buying exclusive items like new Tap Fish fish breeds that aren't available with plain old Tap Fish gold coins. Normally, you have to pay real life money to get fish bucks, but several tricks  can net you free Tap Fish bucks. First, launch the game and click the fish buck icon in the top right corner of the screen. Scroll down and press "Free Fish Bucks." Your  browser will launch and display several ways to earn free fish bucks. Tap each option to get your free fish bucks. In the past, Tap Fish cheaters have used the refer-a-friend feature to spam people and trick the game into giving out hundreds of free fish bucks, but that code has since been disabled.
Tap Fish Tips: Download Tap Fish 5 for Free Fish Bucks
Already have Tap Fish? You can get free fish bucks by downloading Tap Fish 5, the second iteration of the game, through Apple iTunes. This Tap Fish cheat will give you five free fish bucks, which is worth approximately 50 cents, as of April 2010.
Tap Fish Tips: Use Inventory
Save yourself fish bucks and your hard-earning Tap Fish gold coins by using the inventory feature for your decorations. Trick the Tap Fish game into giving you bonus free gold coins when you sell your fish by filling a tank with decorations. Instead of spending your gold coins on more decorations for each tank, simply move the decorations between tanks using the inventory tool. This will give you thousands of free gold coins, although it can't cheat your way into giving you free fish bucks.