I have a 2.5 gallon glass bowl that didn't come with a lid. My fish is elderly and I doubt he would jump, but I want the peace of mind of knowing he can't escape. How far can they jump and how much lower from the top can I keep the water level? The bowl is only half full now because I'm nervous about this. I doubt he could jump that far, but I don't know for sure and it's better to be safe then sorry.
Also, I have a towel that covers the back half of the tank. I originally put it there because I was afraid he would jump, but he seems to like it and he has calmed down a lot since I put it there. Is there any reason that it could be bad? (I know it's a stupid question; I'm a paranoid fish mommy.)