depends on the fish. the bigger guys, well, i can just look at the tank and see them. the bettas, i check every couple of days, check their fins and such. the puffer i check daily, as i seem to have had a bad batch and have been losing them one by one. i check the hatchetfish daily, i lost some once to a rather hungry fish and have since just become paranoid about them, even though the offender no longer resides in that tank (my hatchets are some of my fave fishies), and i check my livebearers once of twice a week to see how they look, look to see if the females are preggars, etc. the plec, well, i see her when i see her and don't worry much. when i do water changes, i'll do a quick inventory of the rest to be sure i have everyone