P. S. Home Depot, Lowes will not custom cut a piece of hardware cloth for you. If (and when) the help there figures out what hardware cloth is, they are more than happy to sell you a roll 40' by 12'. Only the small stores will cut it to fit your fish tank. Plus, I don't think the piece you buy will cost over $1. If you want to go the extra mile, paint it with some white spray paint made for outdoor metal.
I know about this b/c as a child in the 60s-70s, my Mom decided to raise a shoal of swordtails. Talk about jumpers! She bought a $7 fish (lovely b/w speckled one) and it jumped into a waiting cat's mouth! Another jumped into her handbag- she didn't notice it until it was waaaay too late :0( After these 2 incidents she used the hardware cloth with a large rock on it for good measure. (That was a $7 fish in 1972 prices. She was not pleased with the cat. But, what's a kitty gonna do?)