Oh NoOooooooooooooo NOT AGAIN

[kien]

Your fish ramming into a plastic mesh will be fine. They will bounce back into the tank like a reverse trampoline. My fish do that constantly. I have a very busy tank with a lot of skittish wrasses. Some fish will jump even if they are startled by lights turning on, or a sudden thud of someone's foot on the floor, or a door closing. That glass box of there's is quite a good amplifier of sound.

As Red said, goby's and wrasses are notoriously skittish and their natural defense mechanism when they are startled (for whatever reason) is to bolt in the the direction with the least obstruction. In the tank that direction is usually straight up.

I wouldn't say that you are doing anything wrong. Sadly, it happens. In fact, I've seen all manner of fish jump out of the tank from tangs to angelfish. Fish like to dart in all directions. And if we want to drill deeper into this discussion (pun kinda intended), our tanks are not actually that deep. We may think that 24 or even 30" is a fairly deep tank but in the wild, that's a shallow tide pool that many of these fish would never naturally swim in, unless they enjoy living on the edge..

As for the mesh, try UFA up in Airdrie. They sell farming supplies and equipment specifically.