Responding on the cory issue.
Fish in aquaria that rarely see "traffic" will be much more skittish than fish in aquaria that have people passing by regularly; this sometimes depends a bit on the fish species, but corys I have found are definitely like this. This is why when you first acquire fish, they may be out a lot, but as they settle in to a "quiet" environment, when you approach they feel threatened. My 7 tanks are in a fish room at one end of the house, and they see no one except me and only when I enter the room. The corys in particular will all disappear as soon as my shadow or image appears. After I sit in front of the tank a few minutes, they begin venturing out. If I move or get up, they scatter behind plants and wood again.
This reminds me of something I read years ago. An aquarist with discus in tanks in his fishroom always wore a white lab coat when working on the tanks. Whenever a visitor entered in dark clothes, or if the aquarist did not wear the white coat, the discus would disappear behind plants, trembling. They recognized the white coat, knowing there was no danger. Fish instincts are quite remarkable.
One trick to always see them is feeding. Always feed around the same time each day, preferably in the morning about half an hour or more after the lights come on--no sooner, as it takes some fish a while to get accustomed. For the same reason one should never feed just before lights out, give them an hour or more to digest the food naturally--except nocturnal fish, obviously. Fish fed at the same day each day quickly learn this, and their "inner clock" tells them when food is likely to appear. When I first moved to my present house i was still working, and had to leave well before the tank lights came on in the morning. So I switched to evening feedings, as soon as I got home around 6 pm. Within no time the fish all picked up on this. On weekends and days off, when i entered the fish room during the day, they reacted normally. But around 5.30 pm they would always all immediately come to the front top of the tanks when I entered. Their instinct told them this was the usual feeding time. This is one way to "see everyone" daily.
Another trick is to sound the dinner gong
. When I feed my fish I use a plastic 1/2 teaspoon as it is safer than shaking food out of a container, and I always gently tap on the tank frame with the spoon, always. When I do, in goes the food. Within minutes, the "hidden" fish like corys, Farlowella, Whiptails and pleco are scampering out and around the front, looking for the food. Even if it isn't in the tank yet, the "gong" tells them it should or will be, and they are prepared. Doing this, I sit back and can easily count all the 30 some corys which during the day I may never see, at least not at once.