Feeding your fish in ME2....

Usually, it's best to err on the side if caution and do it after every mission or when you leave the Normandy. I think you can get away with forgetting once, but most people flirt with Kelly so that she'll take care of it. Since flirting and dining with her doesn't affect any current romances, it's a popular tactic.

In order to keep your fish alive yourself, you must feed them between every main mission or side quest or optional mission that takes place on a planet (N7 missions and recruitment and loyalty missions).  You do not have to worry about feeding them between each little side quest on the hubs; but it is a good idea to also feed them before leaving the ship to go to a hub.

In order for Kelly to eventually offer to feed your fish for you, you must have flirted with her from the very first conversation you have with her and continued to select the flirtaceous options in any subsequent conversations.  Miss one flirt (regardless of Shepard's gender), and she will not make the offer.  After your recruit Samara and you talk with Kelly, Shepard will have another option to flirt, Kelly will suggest they take time to really get to know each other, and Shepard will suggest dinner in his/her cabin.  It is the next conversation after that cutscene and conversation sequence that will yield the offer of feeding the fish.  Once she makes that offer, you can just forget about feeding them yourself... but you can still feed them if you want to (i.e. you can't overfeed them).  If you're past that point in the game and she hasn't offered, it's very unlikely that she will offer.

For me, having Kelly do this task is mandatory... Without her doing it, I have never successfully managed to get my fish to stay alive through to the end of ME2 since I have always forgotten to do it somewhere along the line and the fish have died as a result.

Ugh, feeding the fish. The little buggers always die on me. No matter how much I try, I always forget to feed them one time and bam! Dead! Little floating carcasses reminding me of my failure as Shepard. If I can't save my pets, how am I going to save the galaxy? HOW?

Anyway, Kelly isn't always the solution. I aim for roleplaying first and foremost, so often my Shepard won't flirt with Kelly. It doesn't help that I find her personally unattractive with that in your face feelgood shtick she has going. I knew you'd be there to catch me if I fall! UGH!