Wait, is the plastic extending down into the water?
If so, be really, really careful. If the edges are at all sharp, or if your fishies can *possibly* try to get in there, the can really, really hurt themselves. I had one get stuck in a plastic cup once that I'd cut a hole in to float some plants...I had no idea he could even attempt to get up in there. I found him half in/half out, and with a seriously huge slice on his side once I got him free. He lived a couple of weeks before dying of his injuries. :(
I can't tell from your pictures (as the plastic is clear) so maybe this is not even an issue and your fishies couldn't get in there anyway, but, I thought, better safe than sorry. :)
Hopefully this will never happen to you, but I wanted to mention it just in case! :)