Feeding Frenzy

Play Stage 21: Deep Water Dreams

Focus on eating the school's of small red fish. When you are close to finishing the level get eaten by a shark or whale. Continue this until you have used your last continue. Finish the level before you run out of lives on your last continue. You can get 1000+ each time doing this.

Tips:
-Avoid the frenzy bubbles when you are in the third growth stage. They can end the level quickly.
-Bite the whale's tail to increase your score faster and get more lives. Just be careful when it turns around to try to eat you.

I haven't seen this posted anywhere, and I don't remember where I found out about it, but this method can get you ~7000 fish per hour.

Play Barracuda Buffet (Level 36) and finish the stage. Before transitioning to the Bonus Stats screen, position your Orca so that it will die by an oyster while the Bonus Stats screen is up - the oysters continuously open and close in the background. If done correctly, you'll see the screen flash while counting up your Food Bank and Score and it'll reset the screen, doubling your Food Bank and Score (since it thinks you completed the stage twice).

This method will work for any stage, as long as you can find a way to die while the Bonus Stats screen is up and tallying your score. If anyone can find a non-Bonus stage that this will work on where the Food Bank can be easily doubled, please let me know and I'll update the solution.

I don't know where they pulled the number 40,000 from, but getting this achievement is sure an achievement. This means that you must eat 40,000 fish and accumulate them in your foodbank. *sigh*. I NOW have it, and to be honest, very few do. You have to have extreme dedication to keep playing this game this long for such a measly amount of achievement points. Here's some basic strategies.
1) Combine this achievement with 10 Hours, Frenzy Fanatic, and Shark Eater.
2) You can do one of two things. You can either just keep replaying through the game or you can keep replaying certain levels. I recommend altering between the two to make it less repetitive.
3) Start by working towards "Shark Eater" by replaying a level with Sharks in it. I recommend one that has "Mines" in it so that you can kill yourself before you beat the level so you can eat some more sharks with your plentiful lives. Level 37 is a good level to do this on. NOTE: YOU MUST BEAT THE LEVEL TO HAVE THE SHARKS COUNT TOWARDS THE ACHIEVEMENT. DO NOT LOSE ALL YOUR LIVES BEFORE BEATING THE LEVEL.
4) You don't get points in your foodbank while playing Time attack mode, so make sure you're in adventure mode.
5) I've heard a lot of people recommending level 21. I've heard reports of people getting up to 800 fish added to the foodbank before completiting this level with their last life.

I recommend using Stage 2: "Feeding Frenzy!", for this achievement.

Playing through the campaign, time attack, and eating 1000 sharks before hand should net you around 13-15,000 fish eaten.

This level takes between 18 and 25 seconds. Add in the 5 seconds to exit to the menu and re do it after completing it and eating 25,000 more fish should be doable in less than 5.5 hours.

There are also no enemies to contend with, so the time spent will be much less stressful.