I wouldn't worry so much about what you write. Over time, a fish is either going to improve their poker game or leave when they become convinced that they can't beat the game. Either way, I doubt chat comments will help much.

As a result, I will sometimes indicate, politely, what was wrong with the play. I'll type "nh". And then follow up with "not sure about calling that preflop raise with 10-7, though; didn't you think I might have Aces?".

I wouldn't do this if I really thought my comments would drain the aquarium. But I also want people to learn about the game-- I don't want everyone to play like Phil Hellmuth or Johnny Chan, but I also don't like playing with a bunch of people who don't know the first thing about what they are doing either, as they can increase variance a lot.

Interestingly, live, I have been asked several times by other players for some tips as I've gradually built up a stack while other players are being felted. And I have offered sincere, though general, advice (i.e., you need to tighten up your game; pay attention to the playing styles of other players so you have a better idea of what they are communicating to you about their hand when they check, call, bet, or raise; and be willing to fold even good hands when you know you are beat). I do understand that those players will get better and will continue to play me, but I also figure that a player that is going to ask me about improving their game is also going to seek out other sources of information (perhaps they will even log onto FTR!) and improve their game with or without my help.