Don't worry people, the cat won't be harmed

I do understand that its natural for a cat to do this, both my cats are outside cats and are always killing birds and mice. I hate it when they do this but theres nothing I can do about it. Actually, we don't feed the birds around here because of the cats, so I guess that helps a bit. And before the "why don't you keep yours cats inside" argument starts up, we never intended for them to be outside cats, but they spent there first few months of life outside all the time so when we brought them home they got extremly pissed off that they couldn't go out, our male even started to destroy stuff.
And yes, there is a lid on the tank, its glass and theres a small opening going along the whole back of the tank, just big enough for our cat to reach his paw down there

So that'll have to be fixed, just don't know what to do with it, don't want it to look junky as this tank is in our living room.
I didn't mean to start a fight